Mil lives about 10 minutes walk from us. We have ds4 and ds2 so life is hectic and we are tired, our relationship is suffering because we hardly get any 'couple time' apart from when the kids go to bed, and we are both tired by then. If we do want a night out we will ask mil to baby sit every once in a while and she will say yes most of the time - but she has never once offered to look after them either for a few hours or overnight.
She does work and has her hobbies so I know she is busy, but I would expect that she would offer to take them on a random day she may be free, without waiting for me to ask.
My family are in a different city so mil is the only family we have close by - AIBU to expect her to offer every now and again to give us a break?