Yes, my sister wanted/wants to keep her 3 cats. Her 4 kids really love the younger one & has had to give up so much already.
is why it doesn't make sense for your mom to be involved in the first place.
Your sister wanted to keep her cat. You didn't want to adopt the cat. No one was in disagreement. If your sister later decides she needs to place the cat, you already have a full house - again, no one who has a stake in it is disagreeing.
reason this seems to be an issue is because your mother is trying to control the situation, and involving other relatives.
Would you feel comfortable telling your mom that this is between you and your sister, and asking her to just let you work it out between you? Because honestly, I think at that point it would become a non-issue.
My mom already says that we spend too much $ in vet bills for my older cat KC.
Again, this is very controlling on your mom's part. I'm not trying to say anything about her personality in general because I don't know her, but at least when it comes to pets it seems like she feels the need to be the boss.
The bottom line is, it's your money, you can spend it however you want.
I also really don't get how some parents think. You teach your kids to be responsible - then criticize them for taking responsibility for their pets? That seems very backwards to me.
You take good care of your pets, and you've done good things for your sister's pets.
Don't let other family members make you feel bad.