well, generally speaking, jag sibs are nothing but coastal carpets... so you will get coastal carpets with that pairing.
HOWEVER - jag sibs from the hypo jag line (and possibly others) have shown there to be hidden genes... here's a male i have from a hypo jag sib x hypo jag sib (no jag morph involved)... the clutch yielded about a 50/50 mix of normal wild-type animals and some crazy pastel yellow sibs like mine below. so... ya never know. the morph is not magical but it hasn't been around long enough to know for sure whether or not there are other things happening along side the jag morph that may be inherited by the sibs. it's quite controversial actually... but the results speak for themselves.
here's my male from a sib x sib pairing:
https://ball-pythons.net/gallery/fil...astal_male.jpg