Why not?
It would be like playing an innocent board game with each other, except naked, on a bed. Your sister would just be an accomplice or an assistant, not of any interest for you.
A sayer of truths
I think you have to get them back into that early mentality of twin-hood that they can't possibly do anything apart from eachother - always together and in cheerleader costumes. .........*cough*Ahem..so anyway
For a guy, sister & sister threesome is a pretty awesome concept. Looking at it from a rational perspective and you realise it is pretty ____ed up lol
Not long back, a pal of mine was chatting to 2 girls at a club and told them He's get them both a drink if they snogged each other. They accepted, went at it in a pretty amazing way much to his delight. He then went to the bar and got them their drinks as promised - 2 glasses of water... They saw the funny side but how he escaped without getting a slap I don't know... Lol
Whats this rational perspective you speak of? Get that outta here. Funny story though, you'd have to laugh when he came back with the glasses of water.