It's a "who your daddy?" situation -- they both hooked up with this girl in the past, she passes away, and they're left to figure out who's the father. No DNA identification (the mom left a note iirc), so they end up living together with the kid. Hijinks ensue.
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Current favorite ongoing series. Lovely art, with good character dynamics throughout. Author knows how to create distinct characters in addition to compelling relationships.
There's a lot I want to talk about with this series, but I'll stop there for now.
Daraku Kazoku-ron