Hello @Supermanslilsis!
First off, let me start by saying welcome to EA’s “Answer HQ”.
Answer HQ isn't just an ordinary message board or support center. It's a new approach to our community experience and a fun way of helping other players or to get answers (from players & experts)and get back in the game!
The Origin account is connected to email 1:1. (One email can be only one Origin account.)
Steam accounts are 1:n. (One email can be connected to many Steam accounts. If you connect Origin -with one of the Steam accounts with the same email-, all your Steam accounts with the same email became connected to Origin. )
If your sister's Steam is connected to the Origin with the same (your) email, then you and your sister don't have two Sims games, but only one copy of the game. In that case, it doesn't matter where you buy the expansion pack (if you go the Origin or Steam).
EDIT: Now I got an update, new, correct information: "For Sims 3 and older EA games on Steam its different it don't use the Origin account to link.
But for JFO currently its using the new Origin account link to steam and only one steam account can be connected to one Origin account and not several Origin accounts.
Don't need to be the same email also."
Also, in general, I will recommend buying EA games on Origin & not on the Steam!
Maybe I didn’t understand the question well. In this case, come back with additional questions. I would be happy to explain further!