My sister was like this, when we were younger (in college). She judged my choice of boyfriends, for example. Then, her heart was broken by the man she was going to marry, and after grieving that loss, she came to the realization that life isn't totally under one's control, that we all don't get to make perfect choices, and that no one is immune to tragedy and loss.
Your sister may or may not learn that lesson. It's also possible that this is her crappy way of showing concern. Or that she feels judged by you about how superficial her life is. Who knows.
The point is, be gracious to her, but also to yourself. I chose not to have an engagement ring. AT first, I questioned this choice, and was maybe secretly not so down with it, but as I've gotten older, I'm really glad I did that. My sister got a fancy ring, and then later ended up having to help her DH repay the debt, so...
And if it's really just too painful to look at your sister's Instagram, maybe just turn off your account for a while and focus on the people around you, many of them have so much less, you have to feel lucky about what you've got.

I know I do.