First of all, you're in a really tough situation and I wish you the best of luck.
Does your sister know how much you make? That would irritate me, as well. However, it is standard company policy to not discuss wages in any workplace for the exact reason of what you're going through.
If I were you, I would start looking for another place to work, get an offer, and take that to your manager. If they give you what the other company would (more than what they're starting your sister at) then you can decide to take the new one or leave it. DO NOT mention the wage difference between your and new hire's wages...especially so soon after your sister started working there.
However, if you stay, even with the new salary, you will have to watch what you do and say more closely as you will be looked upon as someone who will always be looking for greener pastures. With your sister being there, it could cause real problems -- not just in the workplace, but in your personal relationship as well.
If you do go to work somewhere else, you will more than likely get more money and then you can thank your sister for helping you improve your situation instead of making you miserable with your current situation...that didn't seem too unsatisfactory until you found out about the wage differences.
It's unfortunate that people have to change jobs/companies in order to be paid what they are worth, but it's a fact of life in today's world. Instead of being smiled upon for dedication and longevity at a company, you're basically taken advantage of and looked at as either stupid, or not very ambitious. That's not true for ALL companies, but the better majority of them.
That's all the "wisdom" I can come up with for now...I wish you all the best.