I raise the fact that you can't break up with family in the same way as an external person. Realistically, it would put a lot of strain on a very important lifetime safety net that could cause a significant harm to both of them. Not to mention the power imbalance from the older one being there for the younger one's entire life.
It's not inherently evil, it's just much too dangerous and would put them in a very precarious situation. It's something I can't imagine being worth taking on the great risks, not just because I'm someone with none of the alike attraction to family (well, everyone else too, as I'm aroace). They are still children. In my knowledge, such relationships are very rarely long lasting.
And regarding having children, while I'm no genetics expert, but I'm fairly certain that genetic malformation a la Habsburg only comes from a long term prevalence of incest. The real immediate problem with children is that they are condemning them to be socially ostracized should their parentage become public and that they would have an inherently reduced family structure (one of the historically prevalent reasons for its avoidance).
But realism is unlikely here, so this all only applies IRL.
Postscript: I bet you can tell I've thought through this before. It's from reading Worm,
My Sister's Best Friend, My Lover, and my anthro class.