A time may come when I can't smile anymore; but don't worry about me, I know what to do. It should convey all your emotions, all your feelings for her, and even your fears and uncertainty. Being with you is like having every single one of my wishes come true. I would rather not tell her brother about it, if it were me.
Doing so can lead to insecurities, jealousy, and even the other person not being happy with you.