I mean this in the nicest possible way, but you can't compare cousins to sisters. Even sisters who don't get on all of the time, are likely to be closer to/choose each other over a cousin, even one they are close to. And many will defend each other to the last, even if they don't like each other.
I can understand completely your cousin getting upset with you calling her sister a junkie. You're understandably upset and angry, but she is still her sister, hearing her called such a horrible slur is going to offend. Maybe she is in denial about her sister being a drug user, maybe she uses but isn't an "addict". Or, she may well be a full blown addict, but "junkie" is still a derogatory term.
You say you lent her £2500 direct, and left the rest of the money at her house. Did you leave £6000 at her house, or did you leave £3500, whih combined with the agreed lent money makes £6000? If the latter, she hasn't "stolen" £6k off you, she's stolen £3500. And can you be 100% certain she has it, and another visitor to her home didn't steal it? Quite honestly, it was silly of you to leave the money at her house, especially if you knew she was a drug user, and even MORE so if you believe her to be an addict. Her addiction (if she has one) was bound to override love and loyalty as what matters most is the next fix.
Did you have a written contract with her over the lent money? If so (and you always always should when lending money, no matter to whom) you could go via small claims to try to get that £2500 back. It's not a guarantee they can get the money even if found in your favour, if she doesn't have it she can just take a CCJ and worsen her credit rating, so it honestly might not be worth it.
At this point, it is clear you are not going to get this money back.
As for family, you cannot expect them to write off their own sister, even if she has wronged you quite badly. Unless she wrongs THEM, it won't be anywhere near as big a deal to them, even if they do agree what she did was completely wrong, unfair, and disgusting, especially given where the money came from. You have to accept that while their view of her might have changed, and they might trust her less, and not think as highly of her, they still unconditionally love her. There isn't a damn thing my sister could do that i would ever write her off, i'd choose her over every other family member i have. It is up to you if you let the fact they are still going to have a relationship, affect you having a relationship with the two remaining cousins.