My Hubby and I looked into it a couple years back. If something were to happen to my Hubby, legally, it would be completely up to my stepdaughter's Mother whether or not I could keep contact with my stepdaughter, or my stepdaughter could keep contact with my son (her stepbrother, who's been her brotehr for the last five years). In my case, her Mother has outright said that if something were to happen to my Hubby, my stepdaughter would never see this side of the family again, and grow up believing it was OUR choice.
Legally, she has every "right" to do so. We've been advised that in the worst case scenario, my Father-in-Law would have to fight for Grandparent's Rights, and hopefully, we'd be able to maintain contact with my stepdaughter through him, although my stepdaughter's Mother could certainly fight it, and possibly win, if she chose to. This is one of my worst fears.
It's slightly different, though similar in my son's case. His Biological Father has not seen him or contacted him in ten years. If something were to happen to me, legally, my son's Biological Father would have every "right" to take custody of my son, if he chose to, though since my Husband has been my son's only Dad for seven years now, it would be be a tossup of who would get custody, probably more of a joint custody thing if my Hubby got him and a full custody with visitation for the StepDad at the Biological Father's discretion if my son's Biological Father got him. (I'm not too concerned about that, though, because my son's Biological Father has said many times that he doesn't want him.)
Basically, StepParents have no rights, and in effect, stepchildren have no rights.