My stepdaughter's mother was diagnosed with leukimia, is it wrong for me to start establishing myself as?

Your role in her life shouldn't change just because her mother is sick. Even if her mother passes you will never replace her...just as you said so don't try to. Your role with discipline should be no different with a sick mother then it was with a healthy mother. If you try to change things she is going to resent you. Her mom is sick...she has enough on her plate right now. She doesn't need someone trying to become a replacement mom. If there are behavior issues then you and her father need to sit down and talk about this with her.