How is bellatrix related to Sirius Black?

Yes in some way, although we are not sure of the exact relation.

Dorea Black (1920 - 1977) married Charlus Potter and together they had one son. These could be Harry Potter's grandparents although it is unlikely since J.K. Rowling did state that James Potter's parents were older when they had him and Dorea would only have been 40 when James was born.

Dorea was Bellatrix Lestrange's great aunt. So Harry and Bellatrix could be second cousins but are most likely more distantly related.