If a child dies without a will does the estate go to the parents or do the parents and siblings share?

I am sorry for your loss.

When a party dies without a will it is called "intestate".  Each state has a set of Intestacy Statutes that govern how the assets of a decedent will be distributed without a Will.

In your case in Montana the following applies:

"72-2-113. Share of heirs other than surviving spouse.

(1) Any part of the intestate estate not passing to the decedent's surviving spouse under 72-2-112, or the entire intestate estate if there is no surviving spouse, passes in the following order to the individuals designated below who survive the decedent:

(a) to the decedent's descendants by representation;

(b) if there is no surviving descendant, to the decedent's parents equally if both survive or to the surviving parent;

(c) if there is no surviving descendant or parent, to the descendants of the decedent's parents or either of them by representation;"

 
How the statute works is that you read from the top down and when you get to a subsection that names a beneficiary that applies you stop.  Here subsection (b) applies and you would never move on to subsection (c) which would include siblings.  So in answer to your question, if there is no spouse or childrens then yes, your Parents are the sole beneficiaries of your sibling's estate.

The Missouri intestate statute is RSMO Section 474.010 which can be found on the Internet and while similar to the above MT statute provides that if there is no spouse or descendatns then it is divided among the parents & siblings, and their descendants, in equal shares.  The probate court has jurisdiction to address this question.