The first time she leaves them alone she assumes it's just Marco freaking out over nothing ("You're having a Marco moment"). She said Marco could tell her about it on the way to the kitchen, but Star interrupts them just a few seconds later and it seems like they were just hanging out.
Next time we see her, she finds Marco spacing out in the backyard, which suggests he just ran off after that awkward moment with Star instead of meeting up with Jackie again. When she asks what he's doing, he says he "just thinking about, uh, stuff" which suggests he hasn't told her exactly what's on his mind yet. She tells him to stop thinking so much and sends him off to go get her some pizza nuggets.
Next time we see them, Marco is feeding Jackie pizza nuggets as promised, both having a good time and not at all acting like they've discussed what happened on Song Day. Marco then ditches her to talk things out with Star.
And the last time we see them is them casually talking again, about two minutes after Star and Marco talk things out, right before Star confesses.
I agree that Jackie would definitely want to help Star and Marco patch things up, especially with such a serious issue that could threaten both their relationship and her own with Marco.
But the only time that happens, she thinks that Marco is the one with the problem, not Star, and she doesn't press the issue with Marco at all. She treats it like it's just Marco being weird and freaking out over nothing like he does sometimes. If she knew that Star had feelings for Marco, I feel like she would have handled it much more seriously and actively, like taking them both aside somewhere and talking it out between the three of them.
Jackie also has every reason to believe that Star supports her relationship with Marco, part of the reason why the confession completely blindsided her. Marco would have had to told her himself that sometime like that was up, and I don't see when in the episode that could have happened.