I know it's an odd and random question but I always wondered about that. I haven't read the books or played the first two games so I don't really know everything about him. I know what you're thinking. "You played Witcher 3 but not the first two games or read the books?!" Believe me, I would love to play the first two games but I don't really like playing computer games because they tend to lag a lot when I play them on my computer and I often get really confused on which key I'm supposed to use for a certain thing and I only have PS platforms so I don't have an xBox to play the second game on. Maybe at some point, I'll play them. Plus as much as I'm a very good reader, I don't always have the patience to read. That sounds really spoiled and lazy, I know. I'll probably start reading them at some point. Anyways, hopefully you can avoid my laziness and just concentrate on my main question. lol
Geralt was an only child of sorceress Vyssena and her husband (I do not know his name), and there is a story about his origin in the tale "The road of no return". He does not have any blood-related siblings, although he calls Vesemir a father and the fellow witchers-Eskel, Lambert, Koen his brothers.
Read the books if you do not like playing games, you'll enjoy it, it won't be a waste of time. In fact, you'll get quite intrested and will want to keep reading, they are not that hefty, on the contrary. :)
her husband is speculated to be the warrior korin. his mother didn't want a child and after giving him to the wolf school she would most likely be infertile shortly after.(witches go infertile over time if they use magic too much)