At the end either Lily or Mei will end sad when the protag decides to go back or stay. There is no happy ending for everyone.
And considering she picked the barrette, she might stay.
@Lilliwyt third solution, send mei to the other world instead, Mc stays with lily, mei gets her onee chan back, everybody wins (unless alternate MC switched worlds specifically to escape mei)
(Sure, maybe they'd have to exchange mei for MC's lil bro, but if I was him I wouldn't mind being the only guy in the world so much)
@Drakan I don't think brind Lil bro is a good idea. You can't know what the Yuriworld will react if they found out you can travel to a parallel world where there still mens.
Thought that the MC was going to take both of her hands and caress them as a loving sister. Making her cheeks light up as she says, "It's okay Mei-chan...."
Edit: ....Nvm