I am just about equidistant (1 1/2 hours approx) between the 2 DDs who have children, One has 3 children 6, 5 and 2 and the other has a 4 month old baby.
Much as I would LOVE to be the granny who picks up from school, takes or picks up from nursery, swoops in for an overnighter in an emergency, it is just not going to happen is it? Third DD has no children but lives near her sister with the baby so can provide some sisterly support but s she works and is often abroad, no childcare.
Of course I am torn! When DH's health was better I could do the occasional late/overnight babysit but he can't be left any more and I have come to the conclusion that you can't be everywhere!
I also have memories of my ageing and infirm parents living 350 miles away and of the stresses of their final years when I was also teaching full time.
No, I think your MIL is the first call on your and DH's time for the present. At the risk of sounding brutal, that is a situation which will not go on for ever, but she needs her family's support right now. Your current arrangement of staying over sounds a good one as 3 1/2 hours drive is too far to be doing that every week, unless you felt you could do two shorter visits a month. It is also expensive, does she realise that? It can be lovely to have Mum round the corner but sadly the way families move around that is rarely the case. Where is your other daughter? Is the time going to come when she needs help?
Don't give in to emotional blackmail, moving is never a straightforward business at the best of times!