For someone who sounds as serious about their photography as your daughter-in-law... someone who has literally worn out some relatively recent model cameras (if what you say is true)...

I DEFINITELY wouldn't try to buy a camera for them!

She probably has a very good idea what she likes & wants and you're unlikely to know the details anywhere near as well as she does. In other words, unless she has specifically stated something she would like to work with, you and your son will more likely than not buy the wrong camera and lens!

I would STRONGLY recommend you and your son instead get her gift certificates from someplace where prices and selection are good, like B&H Photo or Adorama in NYC. Then she can make her own choices and get exactly what she wants,... rather than being "stuck" with what you THINK she would want. Maybe she'll opt to keep one of her current cameras and simply get better lenses. Or maybe she will want a new camera that can use existing lenses from one of her cameras. Or maybe there's another system entirely that she'd like to get into.

FWIW: You mention she travels a lot to take photographs. In that case, a full frame mirrorless would very likely end up bigger and heavier than the cameras she uses now. The camera body might be a bit lighter, but it will need to use full frame-capable lenses which are no smaller and lighter than the lenses for full frame DSLRs. APS-C "crop only" ("DX" in Nikon terms) lenses which she might already have for her D5300 might be smaller and lighter. An APS-C mirrorless camera might be a bit smaller and lighter for travel, provided it's used with native lenses (rather than adapted lenses). But Nikon doesn't make one, so she would need to change systems (and may or may not be able to adapt her Nikon F-mount lenses). A micro 4/3 format mirrorless can be even smaller and lighter. But, again, it depends... For example, mirrorless with viewfinders (if she wants one) aren't a great deal smaller and lighter than the smallest and lightest APS-C DSLR models (such as a Canon SL2, which is one of the smallest).

But, again, get her gift certificates and let her make her own choices. Maybe she wants a Fuji mirrorless (X-mount lenses offer one of the best selection). Nikon doesn't even make an APS-C format mirrorless... only full frame. And they only make 3 lenses for it, so far. Canon makes both APS-C and full frame mirrorless now... but so far there are very limited lenses for both, too (about 8 for the APS-C and only 4 for the full frame... and they aren't interchangeable). Panasonic and Olympus partner in making m4/3 cameras and lenses to use on them, so there's pretty good selection both of cameras and lenses. Sony has been making APS-C and FF mirrorless for quite a while, too... so have a fairly good selection of lenses. But Sony's tend to be more expensive than Nikon's... whose lenses in turn tend to be more expensive than Canon's. Fuji's lenses also ain't cheap. In fact, due to their popularity... they're the "latest, greatest thing".... mirrorless cameras and the lenses designed for them tend to be substantially more expensive than relatively comparable DSLRs (which are now "old school", I suppose.... I'm just waiting to hear, "Hey look, that geezer is still shooting with a DSLR!")

My wonderful daughter-in-law will be 50 next month. Over the last 10 years she has become an excellent photographer with her Nikon 5300 (18-300 lens) and Sony RX100 M1 -- but they are about worn out. Her job has her traveling a lot -- both domestic and international -- so she takes a lot of travel pictures but also many family shots. She processes in Lightroom and publishes Shutterfly books.

Seems like a good time to move to a mirrorless system. Smaller and less weight than the Nikon 5300 would be good. I have read a bunch of reviews, but not sure about full frame vs M43, size, etc.

My son will buy the camera body, I'll pop for a lens. Hopefully a package under $3,000.

Thanks for your suggestions!

Let her choose is correct, now what she needs to do is inquire upon this ignoble assortment of misfits and miscreants as to the best choice for her.
If money is no issue, Fuji, XT-3 please, but glass is $$$ and not many choices outside of Fuji themselves.
There’s Sony, wow, yes great quality, in FF, and nearly as many alternative, no! more alternative lenses than even Olympus. But I don’t like Sony Cameras and I don’t know why. My fault I guess.
Z6 Nikon, hey if ya gotta bunch of Fmount lenses that you wish to continue using, well there you go.
I’ve got bad GAS, chronic it is
If money is an issue, the 4:3-m43 family has the greatest and most cost effective assortments of lenses, and 95% of Fuji image quality