So in gen two of my main family, my girl fell in love with a townie. He and his father died (my girls a vampire so she's still alive), but according to the family tree, his mothers still alive. Considering I'm on but gen 5 now and she's only human this is weird I'd like her grandchild and distant family to meet her until she eventually dies, hah. I knew her traits but sin finder located other Sims the same age, star sign and with all the same traits. Is there any way I can track her down?

I'm not sure you will be able to find Service or Role sims that easily they hibernate and/or the game itself replaces them at certain point (usually aging up). But if you have Late Night apartments with NPC doors she could be hiding in there, again, depending on what kind of a "townie" she is. If your sim is a Resident NPC (Inactive) you might be able to locate her. Assuming you have

Master Controller, see if this works:
From City Hall or a computer>NRaas>Master Controller>Sim>Focus>There is multiple criteria options. You might want to start by selecting Homeworld and that will bring up everyone
Is the mothers picture anywhere in the Relationship Panel? Right clicking will cause the camera to go where the sim is
Where she is showing in the family tree try using Master Controller>Advanced>Reset Sim

I suspect The Master Controller Cheats Module might be needed. I've had that in for so long I don't recall.

I'll have to try master controller, she's not a service npc and rather one of those "homeless"residents, like her son my active sim married and her husband who died (he is grayed on the family tree and his grave is in my family graveyard)

Homeless also might be hard to locate. If you have Overwatch installed with the setting to "Clean up the Homeless" set to true she might be long gone.

It might be a NRaas RelationshipPanel thing, but you can do certain interactions directly from their icon in the friends list. Provided, of course, that you know them. So just go NRaas -> Master Controller -> Basic -> Invite Over from there, and the NPC will come to your house.

Clean up homelessness is set to false. How can I locate her from master controller? She's 'alive' according to the family tree but searching by name yields nothing. Victoria Raymond if it helps, Alberto Raymond and Sergio Raymond were her husband and son.

Most strange, she isn't in the list of Sims to invite but simultaneously she isn't dead so I cannot use the locate grave function. Is it possible she moved out of town since story progression is enabled?

It is in fact VERY possible, and it's one reason why I don't use story progression. After I spent hours giving sims nice clothes and jewellery and putting juuust the right shade and intensity of lipstick on every pig... err... sim in town, I don't want SP moving them out or whatnot.

That said, I didn't say to invite them from the phone. I said from their portrait in your friends list, IF you have Relationship Panel active.

Relationship panel is on but the only sim in my active household who knew her was her dead son, and I tried to invite her via master controller, not my relationships
I think it's probably a lost cause.

First you'll have to determine what type of Sim she is. Use MasterController and choose Sim/Status/Personal from the menu on town halls or computer and search for all possible females, for example. It will tell you whether she's a homeless, or an out-of-towner, a service Sim or whatever.
Depending on what type of Sim she really is, there are slightly different ways to get hold of her or whether it is at all possible. Personal Status will also tell you where she is currently located.
After you did that search, you should run another Status/Personal search but this time select Type Of Sim/MiniSims. If she is listed among those she cannot be recovered.
You can try other search filters, too, like Type Of Sim/Dead, Type Of Sim/Foreigner, /Local, /Alive, /Service, etc. etc.

Ah damn, she's under minisim as an out of towner with aging disabled, hence why she hasn't died of old age. I was able to invite her over, however. Meaning my sim now has a relationship linked to her. Can I enable aging on her again somehow? Also, she has no icon on the family tree or when he talks to her (the icon was there before)
Should I revert to an old save and cut my losses? (I saved before this invite just in case of issues)

Update: Reverted back to the old save, icon is back. Guess I'll leave her to her immortality if her aging cant be enabled again.

Hm, in theory MiniSims should not be invitable anymore as these Sims have lost most of their character data already. They're basically only in the game for family tree purposes, most other information has been deleted by the game to save resources.
Personally I wouldn't spend any effort on that Sim anymore as the chances are that she is broken anyway.

There is some information for Managing Mini Sims on this page.