My son in law can only call me Granny

What to call your new family certainly feels awkward to begin with and is a very individual thing. The examples you give Grammy6 are how I would expect to be addressed in the presence of the children and if they are included in the conversations.
My DH never did call my mother anything at all ever and used to really put himself through the wringer over his total instability to call her anything or work out what to call her except Granny once the children came along. He was always sorry about it but couldn't bring himself to call her Mum as that was his own mother's title and he wouldn't call her what I did which was Mummy - and since she never invited or told him to call her anything at all then he didn't! Neither did my father but that was a different ballgame. I made myself call my new MIL Mum and it soon became quite natural but I must admit that in the beginning it was hard to do it.
DS has a very diminutive MIL and he won't call her Mum because I am Mum and obviously unique and special at least to him . He calls her Mummy as his wife does because that is who she is! On the QT he is not exactly a fan of hers so I think it particularly gallant of him to do so although it looks and sounds very strange for such a tall man to call her that.
Once married, DIL really struggled with calling us anything to start with because she used to call us by our Christian names long before she met and married our son and it no longer felt right. I told her about DH and his own regrets and advised her to just break the sound barrier with Mum and Dad when she was ready and eventually she did thankfully. Perhaps you could try something similar Grammy6.

"an only talk to me via my grandchildren....DG, ask Granny if she would like a cup of tea.......tell Granny how you did in swimming......say goodbye to Granny, BUT Never addresses or speaks to me personally"

He never speaks to you, except through the children? The problem is not your name, rather his lack of speaking TO you? I'm sorry, that must be hurtful.

Some people just seen to find this thing of what to call their pils very awkward. Dh did and so did sil for a while. I would just let it go. Be friendly, etc. but don't try to engage him in conversation or anything like that. As long as you're part of their lives and see your GC, you've got nothing to worry about.

How odd , have you ever asked him why he does that? Are you ever alone with him ? I'm afraid my naughty side would come out on this one .

I had enormous problems around calling my in-laws by their first names. My mum-in-law had been a second mother to me and I would have liked to call her mum but it would have upset my own mother too much. So, on the rare occasions I was cornered I called her Mum-in-Law.

If the problem is that Grammy's SiL can't bring himself to speak to her directly that's a bigger problem. I'd probably put myself out to try to get him alone so he had to speak. It would have become an unspoken little challenge/game long before now!

Dads boyfreind! SURELY NOT

Different situation I know but funny in a way: my husband only once complimented me in 40 years of marriage. We were going to his firm's Christmas Do and he turned to the children and said, "Doesn't Mummy look nice" and I had to make do with that

My son in law has always called me mum, as has one of my daughters-in-law and my second son's ex wife did whilst they were married. There is now another lovely lady in his life and I am more than happy for her to use my first name. My youngest son's wife has also always used my first name and I have a feeling that this may be partly due to not having a good relationship with her own mother so the name doesn't have many happy memories for her, except when used by her darling little daughter to her

I may have got hold of the wrong end of the stick here, but the problem seems to be not that Grammy6's SiL refers to her as Granny when speaking about her to his DCs, but that he never talks to her at all.

So that instead of saying 'Can I get you a G&T, mother-in-law?' he says to one of his Cs, 'Ask Granny if I can get her a G&T.'

If so, yes I think that's weird.

I would deal with it by saying, the next time it happens,
'Oi! I do have a name, son-in-law. It's ** and you can use it to address me directly!'
Nothing like spelling things out to people who have forgotten their manners, or never learned them in the first place!

I had that problem as a daughter-in-law because my husband called her Mam and I couldn't do that for two reasons:
1. She wasn't my mother
2. I just couldn't call anyone 'mam'. The word is alien to me. My mother was Mum even though she called her own mother mam.

Anyway, my dear mother-in-law got fed up with me calling her no name at all so she just told me one day to for goodness sake call her Lilian! So I did and that solved it.

My SIL (to be) calls me by my first name and I am more than happy with that. Worringly my DD has started referring to me as Nana - sometimes even when the GC are not around. As in 'Nana would you like a cup of tea'. I don't mind being called 'Nana' by my GC but when my own DD starts thinking I'm her Nana too - it feels a little odd. Still I like to think she is just being affectionate.

Yes it is rather strange.

My bunch all use my actual name though when my DD and SIL got married, he kept calling me mother-in-law. Not sure why I found it annoying, but I did .... So I started responding with "son-in-law" instead of his name. The penny dropped and it soon stopped.

So if your SIL does the "ask Granny" stuff you could try responding with the "Tell Daddy that Granny would like a G&T" etc etc.

Said with a smile etc .... He may get the message!

I always called my MIL "mum" cos everyone did. She was just MUM but not my mummy !!

My son in law calls us by our first names - which we are very happy with. We much prefer it to being called mum and dad. It did take a bit of time for him to do this though.

M DiL calls me Mother, my DD calls me Mum, of course I'm Nan (DGS has decided now he's nearly 7 he wants to be more grown up, no more Nanny) when I'm with all of them. I always called my in-laws by their first name.

I always had a problem with aunts and uncles once I got past 30. Still calling them Auntie ... and Uncle ... seemed odd when I had children of my own, so I just used to use their Christian names. I only had one complaint, from a German aunt who was furious and I had to give in!

I the the concern for op is that sil does not ever address her directly which is a bit odd. Does op address sil directly and does he respond appropriately.

When I was introduced to my future in-laws I called them Mr and Mrs and that never changed. My own 4 dils call me by my first name.

My lovely DIL does this too. She has never called me by name and it's too embarrassing after five years to suggest she does. She was caught out recently when I was in her kitchen and couldn't find the plates. She had to say my name because the grandchildren weren't there and she couldn't say "tell granny the plates are in the cupboard". I blame myself. When I met her I should have said "call me xxxxx".

My SIL is closer to me than his own mother and we get along very well. We mainly call each other MIL/SIL in affectionate jest

Our son-in-law calls us Mops and Pops which we both like.

My SIL calls me MIL and my DH FIL (I call him SIL) my DIL calls us by our first names.

You're lucky my SIL won't speak to me although he used to always call me by my first name. What is it with Son I laws. Think they feel threatened by MIL especially when they are close to their daughters

In 25 years I never knew what to call my rather fierce and old fashioned MIL.

I found it very difficult ti call my FiL and MiL anything at first - mr &Mrs perkins seemed too formal, first names were just a nono then. ( his sisters H did call them by first names later and they did NOT like it! In the end i managed mum and dad whic is what DH called them as i called my parents mother snd father which terminology DH adopted. It became much easier when they became grandparents. However all my childrens partners call us by our first names and we are happy with that. Though they all tend to use grandma and grandad these days! But they do address us directly hich i think is the real problem in the OP.

Our future SIL calls us Mummy & Daddy when speaking to us and by our first names when speaking about us to others. DD2s partner calls us by our first names. Future SIL always ends text messages to DH with MH (man hugs) rather than an X.