You indicate that your daughter is introverted and that it is difficult to communicate with her. There can be thousands of reasons why she is that way. One of these reasons can be that she feels traumatised by what happened with her brother.
I have no reason to doubt that your son is a wonderful person and that he meant no harm when he was 10. The real question however is how your daughter feels about what happened then.
It is important to discuss the matter with her. You and your son could be relieved to find out that her behaviour has nothing to do with these past facts. It can also be an occasion to express your concerns about her behaviour and work toward a better communication.
If the conversation makes clear that she has been traumatised by what happened in the past, this revelation can be the start of a healing process that will benefit all the members of the family.
It is in any case important to talk about the issue. To pretend that you know nothing and that you aren’t worried can only result in more problems!