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.But at the same time... the dude needs to grow up. He quit an awesome union construction job because he didn't like the rigidity, which is understandable, but what kind of a career is telemarketing? It was the deliberate disregard of future security that got me to talkin'. Idk. Tough stuff.
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.
I know this is a "good" example, but it's good material to work with in terms of the points I'm trying to make about attending to personal value communication anyway. I think it's great in a managerial sense to make people figure stuff out on their own, but I think it's also important that when you're going about this, you give them overt personal support. Implicit in your exchange is that you trust him to succeed, but it's not explicit, which is probably what was off-putting (that, and the "big boy" wording, which has that sarcastic tone that's rather annoying). Saying something like "I trust you to be able to figure that out" or "I'm giving you full responsibility to decide that" would demonstrate personal support and still allow for that learning to take place.An example: I was recently promoted to the store and I told an employee to go to the bank and get change. He's worked for Uhaul two years and we had performed this task once together, but he asked the second time what change he should get. Instead of helping him, I told him he was a big boy and could figure it out on his own. He didn't get upset, but he was put off. But guess what? We got the change, I was able to move on to a next task and he grew a layer of independence.
My overall point being that you're not a bad person, but you're probably inadvertently hurting people and you may even be stunting their success by demotivating them. You can probably cut down on this by working on filtering what you say (silently count to 5 before responding... lol...), thinking about if what you're going to say is going to be hurtful on a personal level, and, if you do make a nasty remark and seem to have upset someone, following it up by talking forthrightly and supportively about the subject.