Is it normal to persue this girl?

Ok, I've met this girl about 1 year ago and from the moment i saw her it has been love at first sight, at first i used to fantasize about being with her then it came true because we starting hanging out a lot more due to our group of friends, however, within the last month and half or so I've discovered she also has a bf which she started dating roughly at the same time I met her. The reason why I've just found out about him is that they never do things together as a couple with our group of friends and she also keeps him a secret. The odd thing is though, from my understanding, the guy has not treated her all to well in the past and to this day. I've asked her sister about her status with her so called boyfriend and she says that she doesn't see much hope in him and that she doesn't understand what the she sees in him because of all the bad things he has done to her(cheating on her and leaving her stranded). So I've been kind of hitting on her with little gestures, subtle moves that are definitely noticeably and she took it in a neutral stance as in she didn't say "stop that I have a boyfriend" but she also didn't say "don't stop" she just took it.... but I'm afraid to declare my true feelings for her because I don't wanna lose her as a friend and to be honest I've never really met her boyfriend but rather just seen him like 6 times in the last year so I don't know what type of relationship they have... can anyone help me with this is this normal or just a crazy obsession?