Why would a mother choose sides with a drug addict child and not a non-addict child?

My mother in law is constintely taking up for her son (my husband’s brother) who is a meth addict and drug dealer. Even when he starts huge blow up fights with his brother who is clean and sober. She is witness to these blow ups that are often about small stupid things and sees that the druggy is extremly violent and pretty darn scary. Heck it got so bad over xmas this year that we will no longer go to her home with her grandkids because we are too scared for our saftey and we don’t want to expose our kids to that life style. So tell me why she is always defending her “poor baby”???