So let me make sure I'm following correctly...
Tsumugi fell in love with Megu after mistaking her breaking up with bullying. Comes to find out three or four years beforehand Megu fell in love and had a relationship with Ryouko, Tsumugi's soon to be sister-in-law. Now Tsumugi is aware of this relationship when Megu lied about it after they got together and had sex.. But she claims she still isn't over her first love?
Dang, I guess Megu can't decide who she loves or something, huh? This story is messy and pretty messed up, considering Megu was around 13 when she and Ryouko were together. Bleh... It's okay writing, but the jumps in time and occasionally logic makes this not a great story so far. Let's see how it lands.