You have no options. More than one wall stopping you. 1. She's underage. You have no choice but to leave it alone. Aside from the size of the age gap, which in and of itself, is not the problem. Just the fact that it's not legal and you can't do anything about it. Move on. 2. She's your relative by marriage. Although technically she's not your blood relative, having anything romantic with a step sibling is screaming for complications and difficulties. Again, move on. Let it go. You have feelings, that's perfectly normal but that does not mean you have to act on them. Don't.