I am going to buck the trend of comments here..
Look at it form the eyes of her sister .. she has fallen in love with a hindu guy, ok maybe would have better to have found a muslim guy but can you really help who you fall in love with
This relationship looks like it is serious as they seem to be maintaining a long distance relationship when they could easily have broken it off
**My personal opinion is You do not dob on (report) siblings which may cause negative consequences
There are a whole host of possibilities of repercussions for your sister by you telling your parents that me and you have no idea about.. violence/estrangement/emotional abuse/forced marriage.... **
How do you really know it will be just be an emotional hurt for your mum and nothing else
Put yourself in your sisters shoes.. would she have done the same to you?
I think it is up to your sister to decide how to deal with this and for her to deal with the consequences, if you take the action of deciding for her you might not be so keen to take accountability for the hurt your sister may feel or any damage this will cause in her future life
I accept it's not fair on your mum, but there have always been secrets from parents and kids.. from who broke the greenhouse window to knowing your sister wants to marry a hindu..
I hope your sister can make a clear decision but you need to support her to decide not threaten..
Totally agree, esp with the bit in bold, have heard lots of horror stories of girls being beaten and pushed/blackmailed/forced into marriages to 'fix' these situations. (Anyone remember that Bengali woman who was taken 'back home' and dumped in a psychiatric hospital when her parents found out she had a Hindu bf, it was in the news a while back).
To the OP pls think carefully before doing anything that can have such serious consequences. It's a long distance relationship so it may just fizzle out anyway and at least there's (hopefully) no danger of her getting pregnant etc. Sit down and have a deep discussion with her, ask her to be honest about where she sees this relationship going, is she going to become a Hindu, would he become Muslim or just stay as they are etc, point out the practical difficulties so it makes her think twice. At the end of the day tho it is her decision, wouldn't panic tho as she's still quite young and these long distance things rarely last long enough to result in actual marriage..