The sisters thing is weird if they look similar. And it's weird over the simple little things, like having a mole high on her inner thigh, and then the sister has one in the exact same place. There can be lots of little physical similarities and you will find yourself having 5 and 10 second zone outs where you will be thinking that it's weird.
Also you want this for 10 years so what you have built up in your head can't be got.
You will see her with 10 years off passion built up and she will see you as an hours pay.
And it won't be normal sex where it can go for on for hours. It will have a countdown timer on it so that's no good.
Sometimes the fantasy is better left in your head.