No share to son.

My FIL earned a property on his own effort and now we don't know whether it is on My FIL name or MIL. We stay in a city and my SIL stay along with them to take care as she is a divorcee. We send every month good amount to them and taking care of them everything of SIL expenditure as well as she is not working . Recently they asked to increase the amount which we could not so they said they will write all property on her name if not also they would do that because they already mentioned us we both are earning. She has none after their life. My SIL used every inch of situation to her favour and blaming us for not keeping them with us or not being with them. We invited many time to stay together as they are solely dependent on him but they were not ready to relocate. My question is:Can they do it? How it makes sense whether it is on MIL or FIL name? They are not willing to write a will now on her name at the same time they said they won't divide it until they are alive. I insisting them to write a will on her name so that we can stop sending money and let them enjoy all the property and we live with our salary without having expectation.