I'm a MIL, who loves her DDIL to pieces, and am still very close to DS (only child of a single mum until after he left home)
Ds still phones a couple of times a week, as does DDIL, we see them most weekends, if they want to, and if we are about.
We dont live in each others pockets, but are very close.
DS is very close to my DH although had left home by the time we married (might be something to do with only being 9 years younger than DH)
They have no children together, although DS has a daughter from a previous relationship, who we have living with us at times, I am also quite close to DGDs mother, she will ring if she needs something, and we do just chat.
We will babysit when they do have kids, but I wont be looking after them like I have DGD, and they know that, mainly because I am retiring from Childminding in 18 months, for health reasons, and will be going back to part time work, but they know that and are quite happy for me to be a Grandmother this time not a surrogote parent.
DDIL is from SA so is away from her family, so I do try to be here for her more.
Just because you have a son, dosnt have to mean you lose them.
Even tho my DS and DDIL are thinking of emigrating to Canada, I know we will still see them, in fact by then we will be living in Scotland and it will be quicker to fly from Canada to Scotland than driving from here to Scotland.
And Skype is a wonderful thing, for the times in between