as dop2000 said, you can't automatically pick instance of the family but you can give the family a couple of variables, one for the weapon it uses and a second one for the ID of the instance.
for example if you already have the sprite that carries the weapon place in screen and you don't create another one you can say
on start of layout for each sprite.withweapon ascending
sprite.withweapon.setvariable gun to "abs ( round ( random ( 0 , 4 ) ) )"
blank subevent under start layout
action wait 0
sprite.withweapon compare self variable gun if equal to 1 lets say
create gun on sprite.withweapon
gun set animation to redlaser.
now when it shoots, you compare which gun the sprite.withweapon has and spawn the bullet/laser based on that.
if sprite.withweapon is shooting
and Pick sprite.withweapon where gun value is 1
action gun create laser.object at gun image point 0,0