For a start, you can use the right word (I'm not just being pedantic here I promise. I'm going somewhere with this)
You're not jealous, you're envious. And while the two words are used interchangeably, they don't mean the same thing.
Jealousy is about sole possession. "I want this thing, and nobody else can have it". Whether its a partner, money, a possession, a lifestyle, it's about having it yourself and no-one else can. It's an entirely negative emotion, it makes you bitter, distrustful, mean.
Envy on the other hand is about equality. "That person has a nice thing, I want one too." You don't want to take your friends boyfriend, you want one who's just as nice. You don't want to steal your brothers Ferrari, you want one too so you can go bombing around a race track together.
If you're constantly feeling jealousy, then you're probably a horrible person. If you're constantly feeling envy, you're probably just in shit circumstances.
So let yourself off the hook a bit. You're not jealous, you're not a horrible person. You don't want to take your sisters lifestyle from her, you just want your own life to be better as well. There's nothing wrong with feeling like that.
Yes, envy is a negative emotion as well, but unlike jealousy it also has a positive equivalent, ambition. You may never have a life like your sisters, lets be fair, very few people do. But you can use that envy, that anger, to drive you, to try and work out if there's anything you can do to improve your own lot. Even if the only thing you can do is slog through today, and then tomorrow, until things start to improve.