Lucky for me, I don't have a sister. If it was my brother and his girlfriend, I think I would tell him that I saw it.. and if i had masturbated to it, it would only be because i thought his girlfriend was hot. Otherwise, If it is your sister and you two don't have some sicko demented relationship, it is better than you don't mention it. If she wanted you to see it, she would have given you a copy or said something like, hey there's something I want you to watch. As much as we like to think our subconscious minds work in mysterious ways, think of the downside. Your sister might not be able to look you in the eyes anymore because every time she did she would be thinking about you thinking about her in the videotape and that's the reality of it. As for you masturbating to it...well a sexy lady is a sexy lady, but if I were you, if she is really cool and comfortable with you, she will let you know about it. As for making your own copy, remember after you masturbate to it, don't leave it in your VCR. You never know who might come over! (think about that for a sec.) (and what do you tell them if they come in and you take the Cassette out, they will ask wut u watching and then if your like oh its SO and So movie they might ask to watch it or borrow it...) Just some food for thought...