Oh so older sister is indeed gay, poor thing is being made to pretend to be gay with the actual person she likes.
I was reading this statement rather differently than everyone else seems to be. It's not that there's any force or coercion involved, it's just that if she's going to be with the person she likes, she needs to pretend she's gay (when she's actually gay). So she's being "made" to do it only by the circumstances, not by anyone else.
After all, when the other girl says, "I know--let's get publicity by pretending to be a gay couple!" her options were either to agree or to give a reason for refusing, which would be:
"I'm uncomfortable pretending to be gay," which sends the message that she's not OK with gayness, or
"It would be too painful because I'm actually gay and in love with you," which she's obviously not prepared to do.
So the reasonable thing (from her POV) is to play along and hope to someday somehow turn "kidding" into "not kidding."