Magic families only like to train one heir. One person to get all of their secrets. Trying to divide them among multiple siblings gets messy. Since neesan was the older one, it fell to her.

The Matou family...their ability to do magic had been lessening through the years. They used to be so powerful, and gradually--nothing. The Tohsakas had always been on good terms with them, so my father gave me to them. So I could be the inheritor of their magic.

I want to think...that he didn't know how horrible they had become. That he had no clue he was giving me to people who would be so cruel to me. But I-I don't know. The next Holy Grail War happened soon after that, and he was killed.