Doctor Who: 10 Things We Actually Know About The Doctor's Family
3. The Doctor's Siblings
When the Master was seemingly killed in Planet of Fire, he pleaded with the Doctor to show mercy to his own... something.
His own what? Own kind? Own brother?! Producer John Nathan-Turner was certainly under the impression that the Master would've said "brother" if he hadn't been inconveniently cut off by dying – which is presumably why the Fifth Doctor is slightly distracted when Peri asks him if he's okay.
Original drafts for the 1996 TV Movie would've revealed that the Doctor and the Master, like Thor and Loki, were squabbling brothers vying for power on Gallifrey. However, subsequent drafts brought the story right back down to Earth, and heavily suggested that the Doctor and the Master had different parents, with the Doctor claiming he was half-human on his mother's side, a detail that has since been widely disregarded.
Whether it was the Master or not, we definitely know that the Doctor at least had a brother at one point, because he tells Martha that he doesn't have one "any more" in Smith and Jones. Presumably, the Doctor's brother perished in the Time War, but he could even have passed away centuries earlier.
The Thirteenth Doctor also off-handedly revealed that she used to have sisters in Arachnids in the UK. It's certainly feasible that the Doctor had siblings that were separated from them when they went to live in the children's home seen in Listen, although from what we learn in The Timeless Children, these siblings wouldn't have been the Doctor's flesh and blood.