Wow... You really have it tough man. Well done tho staying strong all this time!
Your parents are choosing their way of life (religion) over you. If they cannot accept the fact that you gay even after you talk to them respectfully then you should ask a close family memeber (uncle or aunt) to help with this situation. If the worst scenario comes you going to be emotionally seperated from them.
A big spark of hope... Give it some time. Your parents are deeply into their religion and they need time to process the situation. If they loving parents which I think they are based on the way you are (they raised you up, you actually give a shit about your family and their love for you) then they should eventually accept you for who you are. BUT they do NEED time. After all this is their way of life and you have to respect that. HOWEVER you also their child so obviously they got 2 massively important aspects of their lives clashing. (Their child vs their way of life)... and this ain't solved in an instant so yea they need time.
Think about it like a break up. If you really like a girl and you were with her for a few months maybe even years and you'll break up its gonna suck and its gonna hurt because you have/had a strong emotional connection with her.
Similarly, your parents are "breaking up" a part of their religion for you. (Maybe, most likely, hopefully... unless they total dicks - you can tell based on how much love they showed you when you were a child)
But really I can't emphasize enough that your parents need time. If they never show you love when you were a child then I don't think its going to work out (if they were bad parents). If they did show you love when you were a child then I think its going to work out... eventually.
Assuming its the worst case scenarios and you have ACTUALLY bad parents then they don't deserve your love. End of story. Period.
However, I never once meet anybody with parents that were bad like this. I only saw it in movies. So yea I'm confident you have okay/good parents based on probability.
And now for your sister. Your sister needs to realise you got issues of your own and you can't come on a regular basis. When it happens it happens. You can talk to her husband about it and get his opinion.
Its quite good to have a middle man between 2 people that are having a conflict especially if the people arguing are on good terms with the middle man. It makes communication about emotions easier and there is less misunderstandings.
Maybe he can talk to your sister about it and if he has a similar view to your sister then he can express his view. Maybe you can understand it better when he explains it and get more clarity.
But they need to understand your situation and that you have a life with problems.
Well done on going through all this shit in your life and going strong! You overcoming your problems and staying storng. It will get better. Cheers man! Good luck!