your plan actually sounds really solid. telling someone you dont care to discuss your physique works as a complete standalone reply, you dont owe anyone further explanation, family included. forty pounds since march is a big deal, and some people will keep pushing out of genuine curiosity or good intentions, but the boundary can stay exactly the same: something like 'im doing well, appreciate you asking, lets switch topics.' if your sister keeps pressing, redirecting with a question of your own tends to work surprisingly well since most people enjoy talking about themselves, so bring up something from her own life and watch her forget she was even grilling you. different is how you look now, different is how you feel now, and the why stays entirely up to you to share. enjoy the holiday