So… I haven’t done anything. But I know that this little family got a “first time buyer” mortgage for a place in our neighborhood and they are living elsewhere.
They come over on weekends and pretend to be residents but it’s a scam so they can get a better mortgage deal.
They are taking this opportunity away from a family that might be legitimate first time buyers.
Also… they are just going to turn around a rent the place.

NTA… yet. But here’s my advice: learn to interpret others’ behavior charitably for their sake, the way you would prefer that they might for yours, and you will live a much happier life.
For example, I assume everything you said is 100% fact. You said “this little family” just moved in, meaning in the middle of the school year. Have you considered they might be running out their old lease in a different school district; i.e., for the sake of a kid?
This is just one of many equally reasonable explanations that I would entertain before giving myself over to intrusive thoughts of resentment over this perceived, potential injustice.
Even if you can’t think of any reasonable explanation, or know for a fact that this is their actual plan, are you committed to diligently documenting the 183 days of absence needed to guarantee their comeuppance?
Or is it enough to extrapolate that they don’t intend to fulfill their principal residence obligation by end of year to indulge in this righteous indignation?
And finally, assuming these questions are easy and you’ve confidently sailed past each and every one of them, how do you imagine it will feel, in hindsight, having been the karmic force of justice in “this little family’s” life?
Will you be happy that you were the reason they learned their lesson? After all, next time they will think twice before attempting to cheat the system. Surely that would be satisfying. What’s more, they will know it was one of their new neighbors who reported them, and they will remember that forever. It could even be you specifically they think of, if you wished. Would you though?
You have enumerated every reason I turned here to talk about this and get it off my chest so I don’t act in real life. I’m just venting a bit here before I let go.
“Just venting?” From here, it looks like you’re stirring up a mob.
You’re sowing distrust of new homeowners and working class neighbors using the exact same tactics Fox News did to rally the public against student loan forgiveness. You’re even casually spinning everything the way they do.
It seems you are the asshole, after all.
Hmm… me? Reeealleee?