Let's clarify a few things:
First, your MIL is the sole owner of record. Correct?
Second, your SIL is your MIL's daughter. Correct?
Third, your MIL was not married at the time of her death. Correct?
Fourth, when did your MIL die?
Fifth, did your MIL have a will? If so, what does it say about who will inherit her estate?
Sixth, did your MIL have any other children besides your SIL? If so, are any of those other children still alive and, if not, did any predeceased children have children of their own?