Ref. No. 2216/44-2354
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
Your father-in-law can marry your mother and it is permissible in Shariah. The marriage of Samdhi with Samdhan is allowable. Therefore, if there is no other prohibition, then the said marriage is valid.
’’لا بأس بأن يتزوج الرجل امرأة، ويتزوج ابنه ابنتها أو أمها، كذا في محيط السرخسي‘‘.
And Allah knows best
Darul Ifta
Darul Uloom Waqf Deoband