I'm a life long cycling fanatic and literally would not be typing this if it were not for a helmet. Tell your sister the helmet choice is, like wearing a seatbelt, not negotiable. Either the kids can make safe choices for themselves or she can make them for them. If they can't choose safely, then they can't ride period. Besides, all the cool kids are doing it.

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230