My Mother-in-laws house always smells like...?

Mother-in-laws are fickle creatures. I suggest you talk to her about it, but approach it gently and make sure to tell her that you're not trying to criticize her housekeeping. She probably doesn't smell it because she's at home all the time so maybe that's why she ignored it when someone else brought it up to her. Tell her you would visit more often if the smell wasn't so overwhelming. If she likes you then she might do something to fix it.