Honest opinion - Be happy for her.

It rather bitter sounding that you okay with her taking a particular client but not this sibling group. She came and did the job she employed to do. She now moving on. Fact some client going with her, is just way it is. You learned from her and it good to have friend's in this business. Don't be sour grapes and make it awkward.

She not obligated to share her business information with you. Don't put in that position where she have to say she not willing to breach client confidences. That horrible position to put someone in and it never going end well. It not matter who approached who. The situation is they are going. Take higher road and be professional and let it go.