As your parents are Buddhist, as you said, they likely have no religious reasons against it. If it’s cultural it can be changed as they don’t believe there is anything religiously wrong with it. I’m sorry for you, but I really hope your parents will accept you for who you are. My parents aren’t really homophobic but they’d be uncomfortable with it I think which is why I won’t tell them I’m bi right now, but they may find out soon and it’s something I can’t change about myself and hopefully your mum will realise that about you and will realise you’re still the same kid.
Nevertheless I’d recommend telling your mum at some point as she will find out eventually. It’s wrong of your sister to make you feel forced to, as it should be your decision as to when and where. I hope it all works out, and I have faith your mum will accept it, even if she’s initially bothered by it.