For some inexplicable reason, the first reason Serena thought of was that perhaps Charlton was doing it because of her. That he wanted to make her jealous. However, she dismissed the idea as soon as it appeared. For even if by of chance he did think of that, knowing him, it would never be his main purpose.
Thus, she went back to the purpose of his return. Without a doubt, it must have something to do with his father's demise. For like her, Charlton loved his family so much that in the end, he chose them over her.
So, if that was indeed the case, then for him to invite Annalys, the implication was clear to see. Does he plan to woo her with the prospect of marriage because of her family?
In truth, if Serena were to think selfishly, she should be in favor of their possible union because if Annalys were to marry Charlton, it would mean that Annalys would truly never be able to come between her and Geoffrey.