I know Chromium has been identified as a mutagen as well as toxicity reports, but I think you're fine as long as you cycle with with everything else, every 4-6 weeks.
You make a good point about the potential toxicity of chromium in
excessive dosages. There shouldn't be any problems for the average person at 400 - 600 mcg/day, and as previously stated, up to 1000mcg/day is being prescribed for certain people with certain conditions.
Without getting into this too deeply, it's the hexavalent form of chromium (the type found on car bumpers) that exhibits mutagenic properties in a dose dependent fashion, as opposed to the trivalent form, which is the form of chromium commonly found in food. There is no evidence that I'm aware of that trivalent chromium exhibits a similar toxicity.
A single dose of up to 200 mcg daily from a chromium supplement shouldn't be harmful (although it's unclear how helpful it might be for someone with Type I diabetes). However, I think beyond that, the original poster should discuss any additional chromium supplementation with his sister's doctor before resorting to self help.