Try and change it up and not do it the same way. Maybe tease yourself? Call your penis bad names? Do it in a different place like in the shower or kitchen?
I know when kids are younger that they wonder about these things, but I think at 15 and 16 you should just keep those thoughts to yourself. I mean it depends on how close you and your sister are and if you have talked about sexual topics, but I wouldn't just throw it out there out of the blue.
I have closer girl friends and we mostly just talked sex, and I did give them massages, but not really in a sexual way, although I do find them attractive
Girls, how did you discover masturbation? Guys too. I have a feeling girls learn it more through exploring their body and guys learn it by way of other materials (porn, stories, tv, etc.)