What to do if my parents left their house to my brother and I and a set amount of money for sister but the estate does not have any money left for me to give her?

If the estate no longer has any money flow to give your sister, you need to speak with the executor or executrix on what needs to be done. If you and your brother are or one of you is the executor, you can check the remainder of the will document to see if there is an issue with a failed gift. Ultimately, it really won't impact or should not impact the home gift to you and your brother. It just means your sister's gift fails and you will or should need to figure out a way to find that money somewhere else in estate assets.

No, you do not have to do this (though whether you want to or feel you should is a different matter). Bequests in a will are always subject to the implicit condition "if the property or money is there"--that is, no matter what the will says, the beneficiary only receives the asset if it was in the estate at the time of death. So, for example, say your parents had sold their home prior to their death--then clearly you and brother would not receive it. Similarly, if there was no money, your sister does not receive her inheritance. The exception would be if the will itself provided that in the event there was insufficient money to pay your sister, a loan should be taken out against the home, the home sold and part of the proceeds distributed to her, etc.