Why does my mom do this?

My mom only gives recognition to my youngest sibling. She is still in highschool and dances and is good in music. My oldest brother and sister are really good n she graduated with a 3.7 gpa form highschool and is currently going to school for nursing. My oldest brother has a good job kids and a house. I am currently working for my doctorate in pharmacy and bachelors in business management. Well whenever my mom talks to other family members or friends she only talks about my sister and never about any of our recognitions or accomplishments. They will ask about us but my mom just ignore sit and starts taking about my sister. It gets real annoying and instead of telling us we done a good job she criticizes and only looks at the negatives. She is a bad mother who doesn't care about any of us. All she does is do drugs and talk bad about us. I think it's because my younger sibling is my stepad kid and we are fm her previous marriage and she hates our dad. If you ever watched teen mom my mom is exactly like catelynns mom. Why does she do this?