Well, clearly you're not a bad person if you're concerned about this
However, IME, most NTPs could use to do a bit of reflecting on how what you say is generally meant to be taken impersonally, and when applied at a deep, personal level, can be very hurtful. I think because you guys are so low in the Fi department, you don't usually consider the implications of what you're saying in terms of application to the internal value of a person. (Which might be part of why your INFP sister and her friend are particularly vulnerable to this, since they process primarily in Fi!) I say this because I have multiple NTPs in the family, and as such have received quite a dose of NTPness!
My little brother does this, too. He has a legitimate concern because he feels like someone isn't living up to their potential, or is otherwise doing something that is just truly, blatantly
stupid, but instead of voicing that plainly, he'll make cutting, sarcastic remarks about it. I think he feels like the truth of his remarks is what makes them okay to say, but it ends up being doubly harmful because not only is the comment demotivating for its target, but it also comes off as an attack, which undermines the perceived validity of the statement. In other words, why would I want to listen to someone who's just attacking me and making me feel bad? That kind of cutting remark just makes me want to ignore him
and the issue altogether. It might initially have been meant just for fun or maybe to help, but it backfires completely.
As for your example specifically - the other thing that bothers me on a Fi level, is who are you to judge whether or not he is pursuing a worthwhile career? Have you lived his experiences, thought his thoughts, felt his feelings, and fully understood why he made this choice? It's sort of an affront to his worth in terms of being able to choose appropriately for himself. Maybe he
has taken future security into account. I can't really understand desiring to go into telemarketing either, but at the same time, I think it's good that he left a job that was taking away from his quality of life. You can't sustain yourself on something you hate, and you certainly can't sustain a family that way. Your comment is pretty insulting to him on a couple different levels, really.