I don't think you over-reacted at all. And, being ginger makes her particularly prone to future issues. Be a fanatic about it if it saves her future health issues, but don't over-worry about this one week. But yes, do what you must to make sure it doesn't happen again.
Perhaps if you talk to the il's gently about the family history.....and the fact that your wee one is so fair skinned....say something like "I know that you care about her as much as I do, so we have to work together to make sure we don't create future problems for her by not being vigilant now..." (something like that).