Your step-kids are getting it from all sides, but they’re going to be taking it all out in one place: on you.
They have to deal with their mother’s venom, their father shirking responsibility, and discomfort caused by accommodating you as you enter their lives. In their eyes, you are new, you are strange, you are temporary, and you are disposable.
That can change with time. With some patience, empathy, and clear, consistent boundaries, your step-kids will grow to trust, connect with, and maybe even like you.