About the tag and people mentioning it's not precise. It doesn't have to be. I think anyone who doesn't want to read a manga with that tag will not want to read this manga. This is what ultimately matters
Honestly this is like the test case for moral incest. There is no chance for any genetic problems in their children, nor is there any power imbalance at work considering they didn't know they were half-sisters before they started dating
A more suitable question will have been what Mari found in Arisa
Originally, it was nothing. But we saw that during their cohabitation Arisa helped Mari get out of her shell and strive for what she really wants in life.
There's neutron stars dense, then there's the usual manga MCs, and then there's this MC where she literally has a numeric value of how much people are into her but still doesn't get it