樓主 has to get clearance from the mortgage bank re. her husband's liability as borrower.
As what I understand, the mortgagee should be 99 and brother in law after the transfer of title but there should be a process. The bank may have to re-assess the repayment ability of the new borrower(s). Otherwise, the transfer of title cannot be effected (the deed is in hands of the bank).
Would suggest 樓主 to add a condition on the transfer document, the property cannot be sold as long as 99 is still alive - this prevent 99 from kicking out of the home after the sale of the property.
Use the proceeds from sale of the property to support 99's daily living.
99 may not want to stay alone but without $$ benefits, dont think sis in law will take care of her. 99 has to face the reality. This is something she is accounted for.