That person is jealous of your sister. "I would do much better if I just had what she has", which in itself is really disturbing. They believe black people only have one kind of experience in life.
Being black is one of the "oppression experiences" still not really okay to appropriate. You can lie about being disabled, you can pretend to be the other gender, you can even claim you are some nebulous "queer" to get on the gay gravy train. But blackness, with the exception of the egregious examples like Talcum X or Dolezal (who actually got backlash), is inaccessible.
So the spoilt liberals who never got told no just can't handle the fact they can't get this one thing.