Just a question

If you best friend or sister was getting married to someone you know would love her would you still feel upset?

Rebecca does have romantic feelings towards sasha

For me she’s envious that’s she isn’t the one that can protect her.

I think she thinks about Sasha as someone closer than family, but not in a romantic way (I know some people have been pushing this idea but personally I don't think so). She's just frustrated that even though she relies heavily on Sasha, Sasha never opens up to her. If you receive help and care from someone, and that made your life much better, you'll feel like you want to give back/do the same, that she can't do now because Sasha never told her anything.

She is 100% in love with Sasha. In one chapter she literally resents the fact that she was born female. In the context of the scene it's because she can't inherit her father's title, but then she also thinks about Sasha. So she is in love with Sasha. If she only had platonic feelings for her she wouldn't have been this hostile towards Arth. They weren't openly hostile towards each other before he proposed to Sasha. And the whole "If I Were a Boy (by Beyonce)" scene also came after the engagement bit. Thank you for coming to my TED Talk.

Same.
I think Rebecca is way ahead of her time. I think she wanted an independent life for herself and Sasha and I think she is upset they have to rely on a man to protect sasha