I just wanted to vent and understand why i felt this resentful
I get what you are saying OP. When you're working hard and struggling financially, it's easy to feel resentful of someone who seemingly has it all handed to them on a plate. Especially if she has never had a job, even prior to being unwell. It seems that some families never work, and this passes down from generation to generation.
Im sure that it feels like rubbing your face in it when you see her posts on social media, but like others have said, you are only seeing her good days. You know that you wouldn't really want to swap lives with her.
I think you need to block her on social media, focus on yourself and your family, and try to stop comparing yourself to others.
I'm a single working parent, and I'm always skint, but I try to treat myself to something (very!) cheap after payday, so I dont feel quite so much that I have worked all month for nothing.
It is shit, but I think the real problem is that so many people are working hard with bugger all left for anything else after bills and food. That's stressful and depressing, but it's not your mate with MS on benefits fault.
Keep on keeping on - life's hard for everyone, just a different flavour of hard x