Loving it. Out of curiousity, will the MC be able to give others the futa treatment at some point? Free use is every so much fun when the player is on both sides of the fence, and it'd be ever so much fun to see the wrong words "backfire" with somebody as sexually charged as her sister, for example. Not saying she needs to be the submissive, but it'd be keen to see her on the recieving end now and then.
About my only nitpick is how the story often says the player has been going at it for hours with several climaxes, but doesn't really seem to be the part. It's a bit more... aloof. One would figure she'd (the MC) be a gasping, panting, over sensitive wreck from all the action - and her sister much the same. The sheets would definately need to get changed out from a marathon session.
But to be fair, I'm an avid fan of giving as good as you got, haha.