Perhaps your teachers told you ～のだから (～んだから) is incorrect not because it is never used (you already know it's very common) but because you can't simply drop it into any sentence.
While digging around on Google, I came across a very nice PDF published by the Japan Foundation which explains the use of ～のだから. You can read it on your own (it's even got 読み仮名!), but I'll summarize the main points here.
Rules for ～のだから
- Used when both the speaker and listener know some fact, but expresses a strong feeling on the part of the speaker that the listener, although conscious of said fact, does not fully appreciate its implications. (I have also seen this expressed as a mismatch between the speaker's and listener's perception of some matter.)
- The clause following ～のだから often expresses the speaker's judgment, intent, wish, or request.
A mother tells her children, 「10時半のバスに乗るから、早く支度しなさい。」 ("We're taking the 10:30 bus, so hurry up and get ready to go."). The children, however, are occupied with other things and don't do much in the way of getting ready. So the mother comes back and says, 「10時半のバスに乗るんだから、早く支度しなさい。」 ("[Hey, I know you know] we're taking the 10:30 bus, so hurry up and get ready to go."). ～から is used in the first sentence because it's new information and simply provides a reason for the latter clause. But ～のだから is used in the second because the children are aware of the fact that they're taking the 10:30 bus, yet their failure to get ready to leave shows that they don't fully appreciate its importance. ～のだから adds emphasis and serves to pull the listener's point of view around to the same side as the speaker's.
A student comes up to a teacher and says, 「用事があるんですから、早めに帰りたいんですが。」 ("I have a prior obligation, so I'd like to leave early [if I could]…."). The teacher feels offended by this, because the use of ～のだから implies that the teacher should be aware of the fact that the student has something scheduled, when in fact the teacher has no such knowledge. (A better sentence would be 「用事がありますので、早めに帰りたいんですが。」 because ～ので does not presume that the listener already knows whatever precedes ～ので.)
There's a four-problem quiz in the linked PDF which is left as an exercise to the reader.
Context would help a bit here, but just as a guess, this is best filed under the "mismatch between perceptions" category of ～のだから: お兄ちゃん sees his 妹's actions as signs that she might actually like him, but in reality that's not the case, so 妹 decides to set the record straight and make sure she and お兄ちゃん are on the same page with ～のだから (the implication being that お兄ちゃん should be well aware of this fact, but from 妹's perspective, he doesn't seem to be fully appreciative of it).