I may be falling in love with my bestfriends boyfriend??

Okay, hi! (Sorry this is probably going to be long, lol) So I’m 15, I’ve never been in love nor have I had like a serious boyfriend before. Anyway, my bestfriend Kristyn and I have known each other since the age of 5. Her and her current boyfriend Sean have been going out for 5 months, as of today. They’re in love, like the whole “I love you’s” on the phone, in person, etc. They actually went out for about 4 months before the 5 months (she broke up with him because he started getting into drugs, then he got alcohol poisoning because he was so upset, so she took him back because she felt bad). In October he got sent to rehab because of the drugs and he got out literally 3 days ago. Sean and I are friends, we never hangout alone or anything, just when it’s the three of us. Were like the friends who sort of make fun of each other, and kid around a lot.. yeah so we flirt a lot, and it’s ALWAYS in front of Kristyn. I mean I don’t really notice I’m doing it, so I don’t really ever feel bad. Sean and I do have a lot in common, cause I went through one of those drug stages, and hey I’m a teenager, I party. So we have like those “deep” conversations about it and other stuff. The thing about Kristyn is that she’s “goodygoody” and doesn’t do that stuff, so she always gets mad at him if he says a single word about anything “bad”. She gives me the “make him stop/omgomgomg” look and I just smile. I don’t really know why… ? Anywho, she doesn’t know he still smokes cigs, I do though, he told me not to tell her so I didn’t. Should I? MY POINT really, is that whenever I’m around him I’m seriously the happiest ever. And seeing them all lovey dovey just makes me want a boyfriend like that. (EXTREMLY JEALOUS) Maybe I’m not in love, but it’s deff. a big big crush. I just get a feeling, I don’t know how to explain it but I’m smiling typing this! Oohhh I forgot to mention that Kristyn has a purity ring, and I can sooo tell her WANTS her, I think shes completely oblivious. She hates PDA, he loves it. At least from what I see. But at the same time, he’s so head over heals for her, I mean he tells me alll the time, and of course I help him out with advice and all. And Kristyn, she’s so worried that he still smokes, she always tells me “ I don’t think I could do it”. I don’t know what to do! Help? Uhhm so I don’t know if this makes any sense, but comment on whatever! I appreciate it!