This is tricky and long standing. The roots of this behavior can go all the way back to the way your parents treated you individually as children.
Winsome has it correct with the statement 'What others think of me is none of my business.' The sister that is trying to demean you is doing so for her own motivations and since you have a long standing relationship with her, you are expected to react in a certain way. Any reaction on your part even when you try to ignore it, like a change in attitude or a clenching of a jaw or a fist shows that the barb struck home. When you do not fulfill the established and expected reactions this can escalate the situation.
Now you're asking yourself, 'Is this a no win situation then?' Of course not. The relationship that you have with your sister has been established over a long period of time. Any time that you make changes or react unexpectedly interferes with her comfort zone. Again, that's her problem. But it will take time for changes to become the new norm, don't expect acceptance immediately.
All too often in society, if you step out of 'your place' or are more successful than expected, the people around you feel threatened by what you're doing. They don't want to be reminded that they have 'settled', if someone else succeeds it forces them to look at themselves and why they didn't succeed. So they attack and denigrate and try to force you back into their world view so they can feel better about themselves. Some will even go so far as to tell you, 'it's for your own good. We don't want you to be disappointed if you try and fail.'
Be true to yourself and keep reminding yourself that 'What others think of me is none of my business.'