L.B.
S.
i agree you need to get to the bottom of this rirhgt away...the 15 yr old might be having some issues at school, or with friends and you need to sit down at talk to him right away see why he is acting the way he is...you will know i he is lying i am sure ... take him for a burger or something and talk to him one on one...maybe he is jealous of the attention the 3 yr old gets, and is taking it out on him...could be anumber of things but you need to have a heart to heart talk with him alone
good luck
L.