But if you have an MB lease car, them stopping the payments would be breaking a lease you have signed with MB without your or MB consent.
Its no different really to someone loosing their DLA/PIP award at an assessment. MB won’t get anymore moneyfrom DWP regardless how much you have on your lease.
Speak to MB and explain the situation and if you know a rough idea MIL will be in hospital and say you don’t want to loose the car as it would be needed the moment she comes out. They are friendly to speak to and may offer an option, given your MIL’s payment will be reinstated when she comes out.
You could also ask for a price to buy the car if you felt the need. I believe circumstances like tis or someone loosing their award MB are likely to let you have it at a better rate.