Physically YES, legally NO in most countries.
But if the sexual intercourse was consensual and does not lead to pregnancy (the child will be prone to higher probability of hereditary diseases), I see no reason why we must forbid it.
The answer is no. That is just so wrong and is not right.
To answer your question, I can't. I have never dreamt of doing that in the future.
Here in Nigeria, especially among the YORUBA tribe, it is not something palatable and approved — having sex with one's sister is like one is toying with flaring fire 🔥
Well, maybe some people do that, but it's not something prevalent here.
Tips for your next question : maybe you can ask more general than "S3x or a gay thing" because it's NSFW, and musing.io user is from global.
For your question :
Ofcourse NOT, It never crossed my mind.