What 'in' means.
Simply put it means you are affected when something happens to another person, without you having to do anything at all
I.e. if your brother gets a promotion and you are genuinely happy as a result. Then you and your brother are one. However, if you are not bothered, or jealous or don't resonate with him. Then he is not in you.
Christ in us.... Tis bliss.
How do you get in someone? Through your word. The word of another must enter and remain in you... I.e. just relate until you connect ....