Providing living situation information to the embassy!

My Fiancés Mother owns the property he lives in with his sister and they pay the morgage. Do I need a letter stating she doesnt mind that I live with them? Or do I need to ask the bank she pays the morgage too? ( to show immigration)

Hope I made sense.

Yes, you'll want a letter from her stating that she is allowing you to live in her home. You'll also want to provide the consulate with sort of proof from her that she owns the property, such as a mortgage statement.