Your cousin certainly can
and should press charges. The
charges, however, would end
up being against both her stepfather
and the mother -- for the abuse
and for allowing it to happen/endangering a child.
There is a national organization
called Advocates for Children
-- I'm not sure where you live,
but they can help, as can Legal
Aid and other organizations
designed for just this purpose.
Though there appears to have
been no penetration, when it
comes to minors, people take
these charges very seriously.
I hope that you can get her the
help she needs and at a minimum
make it possible for her to stay
away from the abuse.