The Sims 4 LIMITED EDITION for PC (Product Code) usage...

Hellooooo, thanks in advance to you that can answer my SUPER LONG RUN-ON question! (: 

Ok, SOOOOO, a few days ago I purchased The Sims 4 "Limited Edition" for PC at GameStop. let me just say i love The Sims Games! I've played Sims 2 on my PSP when i was younger... Played The Sims 3 Pets on my Playstation, And even used to play The Sims game on Facebook!! lol I originally wanted to get the game for PS3, which obviously wasnt happening. This would be my first time EVER buying a game for my laptop since i just got this laptop aswell, I'm more native to my Playstation 3 system. So i was like yeeesssss this would make for a awesome first PC game!! :D I had my sister (who was equally excited to play the sims 4) deal with the whole downloading and installing bit. She popped in disc 1 of 2. She created an Origin account through her own personal account on my laptop and downloaded and installed the game. Using the Product Code to activate it and be able to play it. It downloaded and installed and she was able to play it. Today i decided i wanted to play so i logged into my computer account and logged into MY VERY OWN Origin account which i previously created when playing The Sims Game on Facebook... a window from Origin popped up and it said:

ACTIVATION REQUIRED
To Play The Sims 4 Limited Edition, you must first activate it on your Origin Account...
(then there is two options) either ACTIVATE GAME or LOG INTO ANOTHER ORIGIN ACCOUNT

I picked the activate game option and then it prompt me to enter the Product Code which i then found inside the games case. I entered it and then it said CONFIRM PRODUCT with a brief description of the game and what to look forward to, i then clicked NEXT and it takes me back to the screen where you have to enter the product code. With a red bar on top that reads, "This code has already been used". I've been trying to google for an answer to see what im doing wrong. But havent found one, but hopefully you guys can help me out.. I've played The Sims 3 on my Playstation and have no problems with that being able to play//share the same game/s with my brothers and sisters, letting them play their own game with the same disc.. I'm just wondering if there is any way around this so that i can play the game with my account without having to buy the game all over again. Why can't i use the product code again? if its on the same laptop... The game is Installed into every account on the laptop, and when you click on the shortcut, it takes me to the Origin window asking for the code.. ☹️( Do i really need a new code to be able to play the game that is already installed to my computer? I dont want to purchase the game again just to get a new code, whyyyy would you guys do this? I dont see why you wouldnt be able to use the same code on multiple accounts that are used on the same laptop... Do you sell product codes seperately? for people who bought the disc and downloaded the game already? this is really lame.. :/ Oh ya and what's the second disc for? TY for your time. ~maggiieee