Short version: HELL NO.

Long version: having sex with a partner's sibling is something that a lot of people think is a good idea.

It REALLY isn't. I haven't dealt with this from experience, but the thing about being caught having sex with another person is that it can lead to LOTS of issues in both the relationship you are in AND the relationship between the other two people if there was one already due to the feelings of betrayal that come from it. Siblings typically have much closer relationships than they do with the average person, so this usually makes those issues worse and makes them last for far longer.

Basically, unless BOTH siblings come to the idea of sharing you on their own and without any prompting or floating of the idea on your part, don't do it. Even then, I wouldn't recommend it unless you are VERY certain that both siblings can handle issues like jealousy maturely and won't regret the decision once they've had experience of it in practice.

I do have one question that, while admittedly an assholish action to do, does strike me as a valid one: have you ever debated leaving your girlfriend for her sister? I don't personally endorse this normally (because it leads to a lot of very uncomfortable situations and paints you in a really shitty light if you do it, to say nothing of the fact that I find the idea of a 27 year old dating a 20 year old to be a bit on the iffy side), but what you've said does indicate that you click better with her than your girlfriend, so...yeah, is that an option that you've ever considered?