What Causes Bright Red Splotches On The Legs?

I have a 14 year old daughter (5'9" and 115 lbs.) that gets bright red splotches on her legs when she is in the sun or gets hot. It is not raised and it does not itch. It is very obvious and the kids ask her about it. She does not have any other health issues at this time that I know of. I have another daughter who is 10 that has positive ANA titers of 5110 (no diagnosis yet) and a 12 year old son that has juvenile arthritis. Should I be concerned that this rash on my 14 year old could be auto immune related?