My confusion mostly stemmed from wondering why the mother would think/know there was a cool teacher, so I think your explanation makes sense.
だって also made things confusing, since it could be だ followed by って or just だって, which seem to mean very different things.
I guess we’ll have to disagree on that one. I just read it the だってさ as “(someone said), huh” sort of meaning. If you say だってさ to start your thought, sure, an explanation is coming, but I don’t see that as what’s happening here.
Like this:
“Note 2) You might have heard/seen some people add さ ( = sa) after って ( = tte) in a very casual reported speech.
Ex. マギーは今日、来ないんだってさ。
= Maggie wa kyou konain dattesa.
= I heard Maggie was not coming today.
Ex. 明日はいいお天気だってさ。
= Ashita wa ii otenki dattesa.
= I heard the weather would be nice tomorrow.”
From http://maggiesensei.com/2012/05/07/direct-indirect-speech-＆ってtte/
Either way, I understand the general meaning of the conversation now, whereas before I was completely lost. So I’m fine with that. 
No, that’s what I meant. In your case, the speaker is informing the listener(s) of something they heard. And the mother cannot be informing the daughter in this case.
(But yes, since @seanblue is fine now, I don’t think it really matters)
Why are you so sure of this? .-.
Doesn’t it seem more likely that Maho knows about her own teachers than the mother knowing?
The speech bubble does point to the mother, but it’s still a bit weird for her to be saying that.
But parents usually know the teachers, too. I thought it was like that everywhere?
Even easier since she’s just commenting on his demeanor, or attractiveness. Not really about how he is as a person, so a cursory glance would suffice.
The speech bubble is pointing to her, and it’s something she could say, but surely she isn’t.
Is what confuses me here. 
Also like QuackingShoe said, Maho could’ve mentioned it before.
But it’s reporting indirect speech. Why would Maho be reporting what someone else says when she knows her teachers first hand?
“I want to marry teacher”
“Ah, I heard there was a cool teacher”
“Yeah but he has a wife and kids and…”
だって can be because; after all. 
There’s a cool sensei, after all.
(which still is something the mother is saying)
Or that I guess.
That ^
I said already, but it’s unclear to me whether it’s だって (because / after all) or だ＋って (quote).
Oh, and that’s made me realize why we are talking past each other.
I really thought (think?) だって+さ (the same way we can have でもさ, なんかさ, and so on)
= ‘cause, ya know, … (so you need something after)
But it could indeed be (I misunderstood the post from @QuackingShoe) だ＋ってさ = “oh yeah, I heard that…” and then there’s no need for a follow up, and it would be a valid reaction.
I still feel like the reaction of the daughter is strange in that case, but it’s not impossible.
Hrmm, I guess I will need a jocker and ask a native.
So ok both those constructions are the same says my local native speaker. It means “because”, but based on something you heard.
It is indeed the mother talking, but explaining to Shuu instead of the daughter. So basically, it implies that the mother knows what the daughter (I should really get back to read the manga and check her name) is taking about. I dismissed that possibility, since I thought it was really BS.
The second thing that tripped me up is that I don’t see why the mother would cut in the conversation to explain something to Shuu, but that’s what’s happening here 
Finally, it cannot be the daughter talking because of the って
Okay, thanks for the update. I guess this round goes to @QuackingShoe haha.
P.S. The sister’s name is Maho.
Yes, that pushed me to actually start reading the manga again, since I have to give them back in a few days 
I’m actually feeling super down that I could miss something like that after so many years of studying Japanese (and living in Japan). Everything I thought I knew is probably a lie.
This is probably affecting way more than it should because 放浪息子 always put me in a slightly negative mood… although volume 2 felt more positive than the previous one…
(When I am less lazy, I’ll write down some thoughts behind a detail tag)
From another perspective, it simply means that you learned something new. We’re all here to learn after all. 
Hopefully that won’t always be the case… 
Ah, but it’s a good (?) negative mood. Drama, you know. 
Emotions are a funny thing.