What should i tell my sister about yea that?

i just got a text and my ex ex girlfriend told me wanna have sex i have condoms and my gf read it she said your cheating on me bla bla bla and starts crying so i go over there and i tell her what happend and then she wanted to have sex well start again and she kept and kept telling me to hit her idk so i did and then i heard something but still kept on doing it and my sister comes in with her bf