What to do if i had a dream my sister died, what does it mean?

Here's an idea: It really tells me that you love your sister -- because it bothered you. These aren't prophecies, these aren't fulfilled wishes, they are random ideas generated by your brain. If you want to find meaning, let it be this -- it was to remind you that life is fleeting and precious, and not to let a day go by that you don't show in some way that you appreciate her.