Assalamu Alaikum
and i dont know if they have been through puberty yet, i dont even know if my own brother has been through puberty, i mean hes taller than me growin a lil mustache but his voice didnt change his shoulders are a lil wider and btw they are like 14 yrs of age...so i dunno*and i dont wanna know* but just in case they have even if they havent im going to consider them as being nonmahrams since they will be soon inshallah.
fi aman Allah
w'salaam