Mother in law is caused so much problems?

My mother in law is now widowed before her husband died she was a bitter woman always putting her husband down he was such a loving person. Anyhow she is sly and causing so much agro between me & my husband..... her other son has nothing to do with her she caused trouble with his first wife and now his second. Her daughter is the bee's n eee;s she fusses all over her and my husband well he just there when she needs something repaired/sorted. She turned up to my son's b.day tea and stayed for 30mins left and went to meet her daughter (my sister in law) 30 miles away who also never showed face at my son's party. My husband has made excuse after excuse my friends & family cannot believe how ignorant she is I totally can't stand her or her daughter but my husband still put's them first and I'm alway causing something that isnt there I've had enough!!!