Hey, I think theres only a handful of people who have unlocked it.
I'm level 27, I've retired 6 Sims, completed all the other hobbies and the business, barista, designer and chef careers (currently working on medical and DJ) and 7/17 relationship stories and sadly, no heirloom, I havent even seen it in the store.
But I'm about to retire 2 more sims so I'll comment again if I get it

I'm level 30 and I've unlocked all hobbies (except piano) and all careers and haven't got the piano heirloom. Haven't seen it in the store either.

You have to buy the Fancy Finger Piano Pack for 350 Simcash to unlock the piano hobby.

Me on level 40. Ive completed every single thing; relationships, hobbies, careers. Retired more than 15 sims already but still dont get the piano heirloom. The only way is to buy the piano pack via simcash which is ill never get to. Hmmmmmm

I have the same issue with Kisenosky and Pino :(