At some point you have to do the right thing for yourself and tell her how you feel, and tell her you love her and want to be together. You will regret it forever if you don't.

However, please be aware of the consequences of what's going to happen once you do that. It sounds like she is committed to the "friends-with-benefits" thing, she has made peace with it and the arranged marriage, and she sounds pretty mature and together, someone who's doing what she wants to do.

So once you tell her how you feel the relationship you have now will probably be over as you know it. Because she will be aware you like her, she doesn't seem to have those feelings for you, but she does like you and care for you as a friend and I'm sure she doesn't want to hurt you or make you uncomfortable.

You probably don't want to tell her right now because it may cut the time you have in this thing you have short, but you also don't want to wait until her parents have picked a spouse for her and she's about to get married either. Once you do tell her you have to be mature and respect her decision which will probably be really hard.

On the same note you might be tempted to argue with her or change her mind on arranged marriages and whether they're good or bad & so forth. This is a probably a bad move and could backfire on you. She seems to accept it, and you can't make that decision for her. It seems crazy to me & people in American & the west, I can't believe it's still a thing, but again you can't make her do what she doesn't want to do. I'm sure you probably don't like it. I would probably mention to her one time that you think she should pick who she marries because you want her to be happy. Then drop it. If you push her too much you risk alienating her.

Given the information you've provided I think we all know how this will probably end up. You've probably got a limited amount of time together. Enjoy it. Do everything you've ever wanted to do together. If you have some vacation time, take it and spend it with her. Take pictures. Make memories. Enjoy your time together. Don't look at it as "I'm romancing this girl and I'm going to sweep her off of her feet and convince her to stay with me". Look at it as "I've always wanted to go to this place with her, I've always wanted us to do this thing together" and make that happen.

If things work out for you I'm happy for you. If it doesn't it's bittersweet but that's why you make the good times while you can. Nothing lasts forever, so appreciate what you have while you have it and make the best it can be.