Forgotten manga title. It was story about mc getting a new sister from his parent remarried situation. But it turned out who became his sister is an ex girlfriend from middle school. It make the situation become awkward for the two.
Praise the gods of luck! I went searching for nearly fifteen minutes because I recognized this and I found what is probably it. Mamahaha no tsurego ga moto kanodatta. And if this isn't it, I'm going to go cry a little.
Description sounds about right. I tried searching but found......real life stories of forced marriage...... Edit: Interesting display pic.