I feel very much as ChanelNo5 does. I've never had much input from my family as we live 600 miles away, and came to rely on friends rather than family. As a child, I only had one grandparent, who lived with us for 13 years and was a PITA. (Wonder why I was the only one who got on with him??? )
I confess, I feel uncomfortable with Green's suggestion that we should expect help from grandparents. Presumably, they didn't ask to be grandparents. I think that we can hope they will help, but expecting seems to be akin to demanding, which in turn smacks of duty. Once we are adult I don't think we can lay claim to anything from them, although support is always welcome, of course!
I think there can also be lots of reasons behind grandparents non-involvement. Sometimes they feel that they've done their bit with regards to child-rearing and don't want to change nappies/give feeds/read TTTE books ever again. Or their own lives are so full that they don't have the time. Maybe they themselves have elderly parents to care for. Sometimes they can be nervous about other people's children. (My SIL's mother was so scared of her as a baby that her dad did all the baby rearing - pretty unusual in the 1950's!). They might prefer older children - or are plain not interested because life has moved on for them. They may worry about offers of help being seen as interference, or may misinterpret the signals asking for help.
Siblings without children can be unaware of the stresses and strains of motherhood - how many of us truly understood what it was to be a parent, until we had our own child, even though we read all the books, attended the classes etc?
Tigermoth's suggestion of going softly, softly seems the most likely way to change things but developing a friendship network is the most reliable, IME. Hope you are able to sort something out soon. You can always come to Mumsnet for virtual support, too, Keziah.