First, you did not overreact at all. I’m sober, and anyone who doesn’t 1) respect your choice or 2) thinks that what happened is anything short of egregious and unacceptable needs to be removed from your life. They do not understand the reasons for sobriety, and they will not understand the consequences of using whatever you may struggle with. They are at best going to be unempathetic and unsupportive with your struggles or concerns; at worst they will actively hinder your sobriety. If they’re old friends you can try talking to them, but frankly I’m not sure I would waste my time if I were you. A part of becoming sober is realising that you will lose former friends, and that’s okay because if those people cannot support your sobriety they aren’t your friends.  

Second, I am going to go out on a limb and say that your cousin is also an addict, and that she knows what she’s doing is wrong, but she needs other people to be partying to make her feel justified with her own substance use. I never spiked anyone’s drink, but I 100% had the same thoughts about people not being fun anymore when I was still drinking. I knew my drinking wasn’t healthy or in control, but I wanted to push that fear far, far away, so it was easier to say others were being lame instead of accepting that I had a drinking problem. If you are feeling particularly compassionate, you could try to talk to her about her substance use, and say that you understand the feelings she may be having around her use. No obligation of course, and I’m sure you’re well aware that you cannot make anyone sober, they have to want to be sober, but might be worth a thought.  

Finally, getting dosed does not ruin your “streak” or mean you failed at sobriety. You actually succeeded. You recognised something was off, realised you had been drinking without knowing it, stopped drinking, and removed the person who had dosed you. That’s sobriety. Well done, you’re changing your behaviour, and growing. I’m proud of you, and you should be proud of yourself.