When I take in hermit crabs, they are often suffering from improper conditions over a long period of time. Sometimes, it just catches up with them and there's nothing you can really do to prevent it. The damage was done long ago. It can be nearly impossible to reverse the condition in a short period of time. It's a balancing act, and often hard to win.

Don't feel like it was your fault. Had it had good conditions to begin with, this never would've happened. Can't fix the past, and sometimes that is just too much. Even altering their environment and diet slowly isn't enough for them. Sometimes it isn't quick enough, but too quick does even more damage. There's no solution. Sometimes they go downhill immediately after the 'rescue', and sometimes months down the road.

You did the best you could, given the situation. Perhaps next time your sister brings up the topic of getting an animal, you can talk her out of it.