If biological mom and step dad buy house and biological mom dies, then years later step dad dies, is stepchild entitled to that home?

Depends; we need more information. If stepDad does not adopt step child, then she would not be his heir unless he so stated in a Will. But depending on how title to house was help, on mother's death natural child might be entitled to part of house [would not if help as joint tenants with right of survivorship, but might if held as private property or community property law]. You need to speak to a probate attorney; they will likely give you a free 15-20 minute meeting where they can tell you what they can do for you.