Not a big deal. Name me a time when Sif and Heimdall being siblings ever played a significant part in any Thor story. I've read about 2/3rds of all the Thor comics ever published and I can't think of one. They can comfortably do away with the sibling relationship and keep the core of each character intact. Elba's a great actor so I'm cool with it, whatever the reasons for casting Heimdall as a black man.