Your step-mom Roxi has found some very interesting Internet history on the laptop, she knows it's not your dad so it must have been you. She noticed that it was all solo female masturbation videos that you were looking at so she was wondering if you would be up for a bit of fun with her. It's pretty clear she isn't getting what she needs from your dad so you are more than happy to take some wanking lessons off an older woman even if she is your step-mom!