Oh minimo ! Please do not feel guilty. Just enjoy the good weather while it lasts! I had it with my Granny. At home she was very unkind to me and said nasty things about me and my father. Then in hospital, she was telling everyone how clever I was and all the things I did for her etc. (I was still at school, she was my mother's mother.) Once back home she was nasty again. I think when people were around she was playing to the gallery.
However, I think in your case things might be different. Your MIL may have realised how good you are, now she is not so strong. Who knows? Being old is strange. I'd like to think that she is trying to say sorry, she did always know you are a good person. Maybe she was jealous because she had lost her son to you? Whatever it is, just enjoy the good times and don't feel guilty!!