What is the expiation for intentionally breaking one's fast, and is it permissible to prevent one's wife from being alone with her brother when he visits if the only reason is jealousy, especially when the brother kisses his sister or sits next to her?

We do not see a need to prevent your wife from being alone with her brother, given your trust in both of them. Such jealousy is disliked by Allah, and it may have a negative impact, as Satan might cast into her heart that you are accusing her and doubting her. Therefore, we advise you to refrain from this action, even if it is permissible.