How do I handle being a stepmom to a child I rarely see or know?

My husband has a daughter from a previous marriage and her mom is having issues and was placed in rehab. She has lived in another state from us with her mother since my husband and I have been together since she was 4. We see her occasionlly and the possibiltiy of having her full time kinda freaks me out because my husband travels alot for work and I am raising my 19month old son alone during that time. we have no family in the area. Not to mention my independence will be threatented bc i pick up and go with my son whenever i want to. I have priveledges to do this as i am a retired airline employee with benefits. This benefit does not extend to my stepdaughter bc she is not my immeadiate child. So, while i understand the situation for what is is without being selfish i cant help but to dislike the possiblity of a responsibility of a teen child put upon me. Am i wrong for feeling this way any other stepparents have any insight or advice?