It depends if you think it's right to come between two people in a relationship or not. Personally I would say no, you should wait to tell her until the relationship comes to an end. If she is in a relationship with a girl I think it's safe to assume that she is not straight, so once everything is clear go for it.

If a girl is not a bisexual and she is dating a girl, wouldn't that make her a lesbian? Unless of course she is a heterosexual who dates people of the same sex whom she doesn't find romantically or sexually attractive... which would be interesting to ask her about.

So, first clarify that, and then you can work up to telling her I suppose. I think it'd be better to start slow about how you value your friendship and how those feelings have been growing into something bigger, rather than to blurt it out.

Sometimes people get the wrong idea and think a friendship is a sham started to get close to someone you find hot. If that's not the case you need to make it very clear to assuade those fears as it is a hurtful concept to imagine.

Exactly I have no personal take on the situation