what if somebody dieds owning you money

You can sue her "estate" for the money: when a person dies, an "estate" is created to pay off their last debts and final expenses, then distribute the remaining assets, if any, to the people who inherit it. You would sue if the estate does not voluntarily pay this debt.
However, first there has to be someone to administer the estate--that is, a personal representative (often a family member) must be appointed by the court; this would be the person whom you ask to pay the debt and on whom, if the debt is not paid, you would serve the papers for the lawsuit. As a brother, you could ask the court to be appointed personal representative; if the did appoint you, you would be the one managing her final expenses and paying final debts from her assets, if any, then distributing what's left to those who inherit. Contact the probate or surrogate's court in the county in which she lived, to find out who is the personal representative or to ask to be appointed it. You would contact the clerk's office.
Note that a "boyfriend" does not inherit unless there was a will leaving assets to him. Without a will, your sister's money, belongings, etc. will go to her family under the rules of "intestate succession" (the rules for who gets what when there is no will). The fact that she said she was giving her assets to her boyfriend has no legal effect, unless she put that into an actual will--wishes or desires expressed not in a will have no effect on who gets property after death. 
Note that if your parents are not alive and she had no children, then her siblings will likely inherit under intestate succession--i.e. you may be inheriting whatever she had, if you are the closest surviving family member.