As always, the first and most important question is "What does the plan say?"
I have seen default beneficiary definitions that include siblings, parents etc.
In a typical one, the default order is (1) spouse, (2) natural and adopted children and children of deceased children per stirpes, (3) the participant's parents in equal shares, (4) brothers and sisters of the participant and the children of deceased siblings per stirpes, and, finally, (5) the estate.
Unless the 401(k) plan specifies that if there is no named beneficiary then the proceeds go to the deceased participant's estate (or otherwise the default goes directly from spouse to estate), then there being children and siblings here, the money might not go to the estate. If the money does go to the estate and the participant died intestate, then you must fall back on the state's intestate succession laws (not a lawyer - is that term only suitable for crowning the successor to a deceased monarch?). My expectation is that "children" almost certainly outrank "siblings" whichever state's laws come into play.