Can anyone help me?

I would be tempted to talk to SIL if you have that sort of relationship with him. He may be able to put your mind at rest or indeed confirm your fears. I don't normally condone going behind a person's back and if he does not feel able to have that conversation you would have to respect it.

Thank you Gracesmum. I have spoken to him albeit tentatively, he says he has no idea what is wrong with her and didn't offer any ideas. I think it was his way of telling me not to get involved, however I am very much involved already.

You do not say how old your Grandchildren are, if they are of an age where you can develop a relationship independently with them, it might help. I'm not suggesting it should be in anyway secretive.....just a little more one to one but making sure your daughter feels happy about it.
I had a very difficult time with my father as I was growing up - he too was very critical and domineering and I was always so thankful that I could have my Grandmother as a good friend knowing that whatever I said to her went no further. Grandma, was not able to actively help me, and looking back if she had I am sure it would have made things a lot worse....taking sides is never a good idea. However, just knowing she was 'on my side' really helped me.

Hilda they are fairly young but I hope I can be an oasis for them during any stormy times.

This sound a bit like some mental health issue or such, but apart from that observation I cannot offer any better ideas.

Does she seem unhappy or is it that she just gets angry easily.
Can she hold a job down or is she like this only at home? Is this something that has got a lot worse since her children were born?
You could try the "Mind" website who might at least suggest ways you could tackle this . I hope you can make some progress.

I have been sonworried about my DiL for several months now. Only yesterday she opened up to me, out of the blue. i just happened to say this heat is making me snappy and bad-tempered and suddenly she was telling me. She is suffering from bad PMT and painful periods since the birth of her last baby. Plus she has two little ones to look after. So for two out of four weeks she confessed she is snappy and ready to jump on anyone. She has agreed to go to her (female) GP Blue if she will talk to you about it before going down the mental health route, that can always be followed up later. I was convinced my DiL was suffering from depression until we had this chat.
Good luck

My comments are also along the lines of the previous Gransnetter are there any cycles to her behaviour because it sounds like PMT to me. When I worked in A& E we often had ladies in who were totally out of control due to PMT. often the behaviour escalates in the few days prior to ovulation and again just prior to the period, especially if not regular.
Try to get her to see her GP, suggest she tries the well known supplements for PMT and see how she does after a couple of months.
As a mental health nurse myself I used to advise the ladies I saw to try this route before getting a mental health assessment.

My GD's mother (son's ex GF) is volatile and has a nasty tongue as well as being a fantasist. But my GD has always known that she could come to me and get away from her mum for a bit. Blue, if your GC are old enough to spend time with you alone, you could be an oasis for them. They need to know what a relationship with a normal adult is like. Is your DD a frustrated person? Does she feel unfulfilled in some way? You say she was a clever child, so is there something she feels she could have done with her life but hasn't?

Thanks for all your responses. I will try to start a conversation about PMT at some point, it may well be relevant.

She probably did not fulfill her potential academically, however she is employed in a field which she loves and she is extremely committed to her job. She is very self critical about her abilities and often spends sleepless nights worrying about decisions she has made in a professional capacity, thinking that she should have done better. I should add that she is well thought of at work which is evidenced by comments made by her superiors but she still doesn't feel 'good enough'.

Is your GD a happy child Annodomini?

I was about to suggest PMT. Living with my mum was a Jekyl and Hyde situation and it was not easy. Looking back as an adult I am sure she had severe PMT and feel very sorry for her in retrospect. But it was not great at the time.

Yoiu are walking a bit of a tight rope - as you say you do not want to alienate her or the GC, as they will need you to be a firm rock in their troubled waters. But on the other hand it is hard to stand back and say nothing.

It is sad to hear how self-critical she is; and interesting to me. One of my DDs was like this; and I blamed it on her education (she was the only one of our family who was privately educated) where I felt that they were solely interested in academic achievement and not their happiness and general well-being. However she has stopped this in the last couple of years, since she has been treated with an anti-depressant. It really has transformed her life. She is confident, has far fewer migraines and sets herself more realistic goals. It became clear that she had had an underlying depression for a very long time. It is a joy to see her enjoying her life and her family. It may be that your DD is in the same boat.

I wish you well in dealing with this problem.

Yes, Blue, my GD is a happy adult now - graduating this week. She is on speaking terms with her mother, despite some outrageous behaviour in the past and I suppose I will have to be nice to her too on the 'big day'.