Why some step parents turn kids against their own mother

When i talk to my kids. The step mother is always in the back ground running her mouth. Tell my daughter to say things to me when she shouldnt speak to me in that manner. Ive raised my kids without their father for years of their life and the first chance i give him the option to be in their lives they rip them from me and its been hell for a year..my daughter went from being happy to miserable always yelling at me and being hateful to the point i dont want to talk to her but i love her and my son very much. It pains me to know they turned my kids from me in a year 😒😒😒😭😭😭😭