please read and give me your opinion really stressed?

[Blah B]

i have been dating this girl for five months now and i really like her. Before we hooked up i had a one night stand with this girl she knows and he older sister lifes with this girl to. Well this girl is pregnant and has been saying that it is between me and her current boyfriend. My girlfriend told me her sister told her that the girls cousin told her sister that this girl told him that she was sure it was not mine. Sorry for being long and confusing, BUT anyway so i asked this girl about it and she told my girlfriends sister about it and they both started ragging on my girlfriend calling her a lier when she has no reason to. And ragging on me when all i did was ask not in a mean way either. So they said that we need to grow up and quit causing drama cause those two would never do that to each other. When a month ago my girlfriend sister lied to her saying me and this girl talk dirty to each other and it would make her mad if she could see the conversation. My girlfriend knows she was lying and i told the girl what her sister said and she was furious saying she is a lying ***** but believes her about this situation me and my girlfiend told them both they were causing the drama i was just curious if it was true i think i have a right to know and just told them we were sorry and have not talked to them sense. ARE WE IN THE WRONG HERE!