Wow, you may have had "good intentions" here, but a lot of it screams to me that you just wanted to start something. There are plenty of other ways to rescue an abused cat besides breaking and entering and a simple Google search could have told you that.

Also your examples of abuse are very vague and not really connecting, imo. There are some cats who, no matter how healthy they are, will always look smaller. If you can see skin hanging from it's ribs, that's one thing, but I have to say I am not getting that impression. Your claim that she forgets to take care of her cat seems off as well, seeing as you live separately so you wouldn't be able to observe daily activities. Not only that, but if she is having such a severe reaction over losing her cat (a part of her family that she is showing she loves,) I doubt the same person could sit there and not pay enough attention to feed it.

If you want to cut off your family over this cat situation that you jumped the gun on, so be it. I just think that if no one else is seeing what you are seeing with this cat, maybe you should reevaluate your perception and actions.

All that aside, I could be super wrong and this cat could have actually been in real danger. But I once again urge you to think that there are multiple organizations that could have acted in your place. You could have talked to her or your family about the treatment of the cat beforehand. But you did what you did so you have to deal with the backlash.