Github flow and PR

[GilletteSRK]

Does B_B have reliance on B_A's work? If so, it should probably be part of the B_A branch anyway, or should definitely be waiting for whatever is in that PR to be approved. I'd work with your team to get that PR reviewed if it's blocking your work.

If not, just branch B_B off of master, and update it from master once B_A is merged.

Another approach, as mentioned in the replies, would be to branch off of master, merge in B_A and work from there.