Did I do the right thing?

You were looking out for the best interest of all the cats, kittens and mom. You cared for the kittens when she had no one else to help her, and had her cared for when you couldn't afford it. You made sure the kittens went to loving homes, and we can only hope that they took it upon themselves to get them fixed. Do not think you did anything wrong. You went above what most people would do. Yes, you could have taken care of them yourself, but just because you didn't doesn't mean you are a bad person. You did what you could do, and it was more than most people would have done. That cat is probably very grateful to you, even if she can never show it.