Death by Death Flower!

Is the Sim level 9 or 10 in Flower Arranging? Have you been clicking "Scent with Rare Flower" while having a Death Flower in your inventory? Also, the wiki notes that gifting it to a Sim will age them up; it only directly kills Elders, so you may need multiple arrangements depending on the age of the sister.

If it's still missing after all of that, I'd try the Official Forums, probably the Bug Reports: answers.ea.com/t5/The-Sims-4/ct-p/the-sims-4-en

(Lifestyles are kind of weird... it's basically a reward for playing a certain way so your Sim needs to be doing relevant things to unlock a specific lifestyle. I like the sentiments more although they can be a little trickier and we need more of them.) XD