
Be honest. You don't have to go into detail. Just tell her it's something you like reading. It's fiction and make-believe like any other fiction story. She doesn't need to hear anything more complicated than that.
If it goes further, you can tell her it's a love story. If *you* have no problems with homosexual relationships, then this is a good opportunity to make her understand that there's absolutely nothing wrong with two boys loving each other, that it's no different than a boy and a girl or two girls.
In the end, if she's going to freak out and hate it, there's nothing you can do about that.
On the other hand, you might have a new fujoshi in the making. ;) (Just remember to keep it to shounen-ai. LOL)

Lol I hated hearing this as a child, but it works here: tell her to stay in a child’s place. You’re the elder one here and as long as what you’re doing doesn’t hurt anyone or yourself, it’s really none of her biz. This coming from an eldest sibling btw. And I gave you the nice, expletive free version instead of what I would typically use lol.

Ask her about she thinks it's bad. If she actually gives you a decent answer, ask her if she thinks you're bad for liking this kind of stuff. Again, listen. Ask her if she thinks it's OK to judge you like that. At the end, ask her if it's fair to snoop into private accounts of other people without their consent and if she'd like it if someone did that to her.
The entire point is to figure out what's the cause of this kind of judgements - at 9 she shouldn't have strong opinion of her own yet, which means someone (or something) influenced this. Probably her school or parents. Teach her empathy instead. Will do her a great favor when she's grown up.
Ok so I have a twitter account and don't really pay attention to it so I like any post I like. My sister decides, today, to look up my twitter account and check my posts. She sees all the yaoi posts and shounen ai stuff I liked a while back. She confronts me while we were eating alone, confessing that she saw it all and says I'm a bad person. How do I deal with it?!?!
Btw she's 9 :( and she's not homophobic (I think)