As-Salamu alaikum brother and sisters, is it obligatory to tell someone who's about to sin or who's sinning to stop sinning? Jazakum Allah khayran... May Allah guide us, baraka Allahu feena
The person is doing what is required of them in Islam. Yes it is obligatory, in that it is Fard al-Kifayah.
ولتكن منكم أمة يدعون إلى الخير ويأمرون بالمعروف وينهون عن المنكر وأولئك هم المفلحون
And let there be [arising] from you a nation inviting to [all that is] good, enjoining what is right and forbidding what is wrong, and those will be the successful.
من رأى منكم منكرا فليغيره بيده فإن لم يستطع فبلسانه فإن لم يستطع فبقلبه وذلك أضعف الإيمان
Whoever among you sees an evil action, let him change it with his hand; if he cannot, then with his tongue; and if he cannot, then with his heart and that is the weakest of faith.
If you are aware that another Muslim is about to sin, you should always try to stop them if possible/tell them to refrain from the sin they are about to commit; this applies especially with members of your family.