If you are sexually aroused by your sister... something is
wrong... YOu must be a really sick pervert.
Anyway... because both you and your sis have similar genre, you'll
body will automatically repel her as an object for mating... but if
both of you is been seperated for many many years from child birth,
and you got to know her you could deeply fell in love with her
because your body will automatically mislead your bro-sis affection
for love.
Meaning your body can tell who is your close relative and it will
automatically have an affection for the person or repel against
it... sometime can be misleaded into something else if you wasn't
thinking right.
I guess in this case the boy is doing it out of curiousity, fun and
thrill rather than affection for the sister. The thrill of touching
a breast... He has affection for the breast but not the sis.
Nevertheless its still morally wrong. If he wasn't taught right he
might do something worse in future.
