Here's the text from the law:
§ 898. Private sale or disposal of firearms, rifles and shotguns. 1.
In addition to any other requirements pursuant to state and federal law,
all sales, exchanges or disposals of firearms, rifles or shotguns shall
be conducted in accordance with this section unless such sale, exchange
or disposal is conducted by a licensed importer, licensed manufacturer
or licensed dealer, as those terms are defined in 18 USC § 922, when
such sale, exchange or disposal is conducted pursuant to that person's
federal firearms license or such sale, exchange or disposal is between
members of an immediate family.
[...]
For purposes of this section, "immediate
family" shall mean spouses, domestic partners, children and
step-children.
I don't see anything about siblings in there -- that may be a gray area.