What a shame, bluebellinmygarden, that dil put a stop to your childminding just because you went on holiday for a while! Does she think she's the only one entitled to a break, now and then? At first reading, she sounds very selfish to me.
"His wife has always been grumpy and moody even before they had children."
This^^ seems key to me. Either she's uncomfortable interacting with others or, for some reason, just your family. As the old saying goes, "you can't make a silk purse out of a sow's ear," so I don't think you and yours can expect too much from her, if that's any help.
"If my son visits he comes only with the GC, yet we say we'd love to see them together as a family and we are given what feels like an appointment time even when taking a birthday present... I've offered to babysit whilst she can go out and meet friends, although she doesn't keep friends, offered to take to and from preschool, for her and me to take the GC out but she never acknowledges these requests... When we used to take the GC home, she made herself scarce and my son took the GC and we left."
Yes, sounds like she's not interested in being around your family. I'm so sorry. You must feel totally snubbed sometimes. Please take some comfort in the fact that she doesn't try to stop you seeing ds and the gc.
I'm sorry, also, that she ignores your offers. Sounds like she's a stubborn person who wants exactly what she wants (once a week, 8-8)and nothing else. If I were you, Id take her silence as a "no" and stop offering - it's no use. But that's just me, of course.
"I'm just trying to keep my family together but probably trying too hard."
Yes, imo, you probably are, but I think that's very natural for a mum. Ds and his siblings are adults,however, aren't they? They can work out their own differences surely. Do you ever invite them over all at once? You can get them together that way, but you can't talk them into a closer relationship. If I were you (and I know I'm not), Id just enjoy my own relationship with each of them and my gc. Also, Id let go of any attempt to connect with grumpy dil on anything but a superficial basis.
(((Hugs)))