why is my sister in law lying and talking shit?

[busymama]

My husband has 2 brothers. 1 married other has a girlfriend. They went out one night With my husbands cousin and her boyfriend. The next day my sister in law told me girlfriend was pissed off at cousin because she sat on her cousins lap and was over protective of him. Sister in law said that's messed cause their cousins and she shouldn't be pissed which is true..... Girlfriend called me upset because sister in law told her cousin didn't like her. I said maybe because you made a scene about her sitting on her own cousins lap. She said she didn't recall that happening; today I talked to cousin she said she really likes girlfriend and she also said that situation didn't happen.Why would sister in law talk shit about girlfriend, lie to her about cousin liking her and lie about cousin sitting on lap?