You said at the start of your post "I don't know if it's over thinking or if I'm over reacting."
I'm afraid I'm going to be blunt, because you deserve the facts that answer that question though I suspect you know the answer already.
You met your partner, who had a history of using weed, and at the start he had stopped and all was good. Then the habit took hold again and he changed into a person for whom the habit was more important than you.
Sure to keep you quiet he made promises - never kept them even if he did in fact believe them at the time, he offered plans, but did not follow though, he even offered you a gift, but the in the long run that does not help at all.
Trying to get him to stop is not "your shit" nor are you "coming crazy". It simply shows a complete abusive disregard for your quite correct point of view and legitimate feelings.
To top it off you live with his sister, who is the same and makes things worse.
I guess there really only a few practical things you can do:
- Stay and put up with it -and hope a miracle will happen
- Leave, by yourself
- Leave with him and see if you can get a change in environment to help plus -
- Try to get your partner to recognize the problem of his addiction and and get proper help -addiction is a hard one and for many partners only leads to more distress and disappointment.
Reachout (for under 25s)
Alcohol and Drug Counseling Online
Are pretty good places to start to get advice. If your partner is willing then he can use them too.
There is also the Alcohol and Drugs service in your state or territory.
Sorry I can't say you are overthinking or overreacting or it promise will all get better soon, I wish I could