This is a few months every couple of years? Even 6 months every 2 years is 18 months of her not being there.
Damn right you're being unreasonable.
Her son is your husband. So I agree with them (regardless of culture) - he has every right to see his mother (and she to see her son) whenever and for as long as they wish.
I have always held the view that, having married her son, my mother in law is as much a part of my family as my father and siblings and my home is their home.
Who gives a rat's arse how much each person contributed to the deposit for a family home that should be as much his as yours? Or is he only welcome in 2 rooms because he contributed a percentage less?
Oh and for what it's worth - I am paying for our family home in it's entirety. And my MiL is due to come and stay with us for 4-6 months later on this year.
I would expect certain house rules (she contributes towards food, does her share of cleaning and laundry) as such a long term guest so if your mil expects to be waited on hand and foot for 6 months I agree she needs to be asked to contribute. I've had the opposite problem - mine would like to do more than I think is fair on her!
Agree with AF - why did you not consider this before marriage?