So sorry to hear about Emma and little Nina. What a difficult time.

Yup, check with the vet first to rule out any health issues. Is either one spayed? If not, discuss that with the vet, too. And get the vet's ideas on how to handle this situation.

I have a routine I go through when I bring in a new kitty. EVERYONE gets a bath. I make sure nails are trimmed BEFORE I give the baths, to save "MY" skin! It is the same reasoning Catty1 gave with the vanilla. They all have the similar scent. It won't change their scent, but they all have the "we are one household" scent. I never tried the vanilla, but I bet that is easier!

They need separate litter boxes until THEY decide to use the same one. Don't change type of litter because you are now a multiple cat household. This is NOT the time to make those types of changes.

Finding the poop in the upstairs bedroom after Nina passed makes this complex. Did Abby do this because she was jealous of Nina too? Or is this a behavior or illness issue unrelated to the presence of another cat? Make sure you discuss this with the vet, too.

Good luck!

Feed them separately. Put food down for Abby first, then for Suki. Always treat Abby first - talk to her first, say her name first, treats to her first. Brush her first.