Can someone please remind me what time Morgan came home on the night she died?
I'm thinking that maybe she typically slept in just a t-shirt, but because she was going to be up for a while longer (and maybe knew she'd be found the next morning, assuming a planned suicide), she also pulled on the comfy skinny jeans (they're stretchy) so that she had some pants on.
Even if she wasn't planning to kill herself, this would make sense if she thought she'd be up for a couple more hours, puttering around, so she changed into the t-shirt she planned to sleep in, added some pants, then would have just pulled off the pants and bra right before going to bed (when I lived at home as a young adult, I always remained fully clothed until just before bed, and didn't leave my room unless fully dressed.) And I run cold, so I always have socks or slippers on at home.
This also could explain why dad remembers that she had on her pajamas - he was half right. She had changed and maybe it was a t-shirt they were used to seeing her in at bedtime.
Even now, as a fully grown woman, when I come home from work I immediately wash my face, change in to my comfy sweats (sometimes leaving my jeans on, depends on what I was wearing that day and what clean laundry I have), then at bed time I take off the bra, socks and pants and sleep in my sweatshirt or t-shirt (depending on the season.) So the outfit actually doesn't give me pause at all, and makes sense in either a suicide or homicide scenario.