In the event that the parent passes away and the house is left to be
inherited, does the debt + child support that (sibling A) owe to the
government / state have any effect on the inheritance share of the
remaining siblings (the other two)?
Not automatically. The default rule, in the absence of a will or non-probate transfer or surviving spouse, would be that each child receives an equal share and the share received by sibling A is then an asset from which creditors can collect the debts owed to them like any other asset of sibling A.
Suppose a creditor than "executes" upon the interest of sibling A and becomes the new owner of A's interest.
(This possibility comes with a nuance, however. If A lives in the property which is a residence, then A can claim a homestead exemption in A's one-third interest (something that varies in dollar value dramatically from one jurisdiction to another) and a creditor can execute on A's interest only if it is worth more than the homestead exemption and if the creditor pays the homestead exemption amount to A who can then use those funds to invest in a different home.)
Then, usually, the creditor would bring a "partition action" which would require the property to be either divided in kind, if feasible, or sold with the proceeds divided equally between the creditor and the two remaining siblings, subject to adjustments for contributions to the property (e.g. capital improvement expenditures or mortgage paydowns) not made equally by all co-owners, if not negotiated resolution is reach in the partition action.
But, if sibling A "disclaims" the inheritance (i.e. refuses to except the gift received upon death) in the manner allowed under the particular jurisdiction's laws, then sibling A is treated as having predeceased siblings B and C. In all likelihood (assuming that sibling A's children for whom he owes child support are still alive), this will cause sibling A's share to pass to sibling A's children in equal shares at the death of the parent, and the creditor will not be able to claim an interest in the inheritance.
@DaleM's answer assumes a will, and in many jurisdictions, a tenants-in-common ownership would be mandatory in the absence of a specific provision in the will to the contrary.
His statement that "In a joint tenancy the ownership is indivisible - while a lien can be placed over it, the lien holder cannot force a sale of the property." is the minority rule. The majority rule is that even if the property is initially in joint tenancy, any creditor's claim against one or more but not all of the joint tenants, severs the joint tenancy and converts it to a tenancy-in-common.
It is also possible in some circumstances, for the will, or the agreement of the co-owners, to waive the right to a partition action, in which case the value of A's interest in the property is reduced to a right to use the property together with B and C, which is much less valuable to an investor (since it can't quickly be converted to cash) and is very disruptive if the right is exercised because a stranger creditor or their assignee can use the house and walk into the bedrooms and bathrooms otherwise occupied by B and C, anytime that he or she wants to do so, without paying any rent.
In the U.S., the only form of joint tenancy that cannot be severed by a creditor is called a "tenancy-by-entireties" which still exists only in a handful of U.S. states and can only be created with a husband and wife as the co-owners (or presumably a husband and husband, or wife and wife, now that same sex marriage is allowed in all U.S. jurisdictions).
UPDATE based on jurisdiction information:
Inheritance law is normally determined by the place where the decedent (i.e. dead person) was domiciled (i.e. resided on regular basis with no intent that it be temporary only). Real estate law is usually governed by the law of the place where the real estate is located. In this case, both of those locations would be Poland. So, the Polish Civil Code as to the real property and inheritances would apply. But, Polish law is beyond my expertise to answer in the detail of my original answer. My answer and both of the other answers are based upon the rules that apply in the common law country tradition, while Poland is a civil law country.
