Mother in law help please

My mother in law has very kindly offered to help my hubs and I pay for a deposit on a house, which is great and so kind. There is one caveat though, that she is allowed to live with us or in the house until her own parents pass away and their house becomes free. Something about this doesn’t sit right with me and I don’t know if I’m being unreasonable.

I feel like this is a way of her having control over us and don’t know how to bring it up with my other half without sounding ungrateful.... what do you ladies think?