Imagine a line running only down the tree connecting two individuals - that’s a bloodline
If you can draw a bloodline from one person to the other, they are of the first’s bloodline. Your full blood siblings are not in your bloodline, though you share all of each other’s bloodlines
Glad to help.
It’s a weird concept outside of inheritance - for example, a royal bloodline could end because the regent dies without children. Because the upstream follows the ruler, you might have to backtrack up the bloodline to find the next heritor, which you’d call a branch bloodline
But in modern life? It’s kinda pointless as a concept. We care about heredity and family, not bloodlines