Boyfriend had sex with his sister when young

Hello everyone, I’m not an English speaker so if I make certain errors excuse me.

So my boyfriend and I don’t keep secrets from each-other. And last week he confessed me something that I can’t get over with, and I don’t know what to think anymore...

When he was young he shared the same bed with his older sister (1 year difference). He told me that his sister touched him during his sleep and forced him to touch her, and they ended up having sex two times. This makes me so sick and I don’t know what to think anymore. It happened when he was 14 and her 15. They don’t talk about it, and have a normal relationship now.... what do you guys thing about?