AITA for not paying a bill I knew nothing about? or legally am not liable for?

I 31f moved in with my now husband 4yrs ago. My bro 26m agreed to take my dog & I was more than happy to pay anything dog related & even some of his utilities if needed. 2 1/2yrs ago he started dating his now gf. Since any vet appt needed she would call me & say my dog has heartworms or something like that (she's NEVER had heartworms) & also say she needed X amount of $ to pay. I've always paid the vet directly beforehand. & I've asked a few times for this to stay between my bro & I & he's never enforced my wishes. All I could do was ask & let it be. Back in May 2022, my dog fell ill. She calls again saying she knows it's heartworms (probably 3rd time claiming this in under a yr) I call the vet scheduled the appt. She takes the dog, dog ends up needing surgery. I pay half the 1200 but 10mins later vet calls and says it's cancer eating away at her, there's no quality of life left. She was suffering. I handled all payments & funeral arrangements even paid for my bro to get a separate urn & prints done. We both loved this dog equally. Problems kept occurring thru the process of cremation/ delivery with gf. She would text me asking how things were going/updates. I would then get in touch & turned out they've already spoke with her, EVERY TIME I CHECKED IN. Plus other things regarding this but I bit my tongue other than expressing my feelings to our other siblings/patents/husband.

Jump to this January. She's texting me that's I left a $600 bill unpaid & need to pay. This is odd bcuz I was never contacted, & I paid everything upfront & it totaled 1400 between vet &funeral. I find out that the day my dog goes in she changes all paperwork into HER name as primary and leaves my bro as financially responsible, there ended up being a 185 balance that went into collections, that with interest now is 600. I contact my bro l, talk out past frustrations about her inserting herself & she goes nuts overhearing the call. Saying she did no such thing, im calling her a liar. All types of stuff. Winds up hanging up in my face while I'm trying to settle things. I text my bro saying I won't pay the balance bcuz she wanted to be involved so congrats. She's involved. We havent spoken since bcuz i want an apology. AITA?