I always say, "relatives be darned, if it is good for the kids, say
something!"  If you saw someone abusing a child, you would say something,
would you not??  So forget family etiquette, the sister is ALREADY not
speaking to anyone..... send her some info with a kind note about how you care
for her children and thought she would benefit from the info.  But tell her in
a way that she looks fine... ie. "I never would have known myself about our
allergies if it were not for someone helping me..."  LIE.... but do it for the
children.  You are GREAT for being concerned!!!

Katherine