Can a Muslim use word "sinner" for another Muslims? Did the Prophet (sws) use this word for any non-Muslim or Muslim? Can a son called his father "sinner" if he thinks he did not take enough care of his mother when she was sick. Or he got married after his mother died.
I have not read anywhere whether the Prophet (sws) ever said this to his fellow companions.
It is extremely important we stress the sin we notice instead of the person who commits sin. We should for example point out to our friends and relatives that what they did, for example stealing or cheating, is a sin and incurs punishment. This is better because we can avoid offending the addressee who may not listen to us if we use harsh words. When the issue relates to our parents and elders we need to be even more careful.