Holy shit, this manga actually got a really good ending.
Even though it's an incest gag manga...
It avoided outcomes like having requited incest romance or having the female character fall out of love. The first is often very awkward, the second is often a betrayal of the characters and the story's themes.
In this ending, she stayed in love, she stayed the protagonist, and her love itself wasn't even directly condemned... so as a fan of incest romance I actually liked this end. She was even so happy and relieved that she didn't ruin her brother's life at the end there... Can't help but think she truly would've been a caring wife if only they weren't related.
The narrative also did a great job showing how she had a screw loose, which in a sense admittedly half-invalidated her love. But honestly that's realistic/interesting, because only someone with a screw loose would basically threaten suicide if they couldn't have raw sex with their sibling. It was also funny as hell (to me as someone who appreciates heavy black humor like Cyanide and Happiness).