“My Father and mother-in-law live with his elderly widowed mother and all three are listed on the mortgage. There is also a sister of my father-in-law and two of his daughters living in the house.

In this instance, who owns what portion of the house, considering that my in laws are the only ones that pay the mortgage?

If the elderly parent was to pass away then what happens to her portion of the property? If she has no will, then can the house be contested by all of her five siblings?”

The mortgage is not controlling in this case. You need to look at who is listed on the deed to determine who is the legal owner of the property. If someone who is not on the deed has put a significant amount of money into the property–for improvements, not just maintenance or to pay the mortgage, taxes, etc.–that person may be entitled to reimbursement when the property is sold (or may not, depending on the circumstances), but is still not in control of the property.

If there are multiple owners of the property, what happens when one of them dies depends on whether the property is held as “tenants in common” or “joint tenancy.” This should be specified on the deed. With tenancy in common, each party’s interest belongs to them and passes on their death in the same way that any other of their property passes: either as specified in their will, or under the applicable intestacy laws of that state. With joint tenancy property, the part interest automatically goes to the other joint tenants, regardless.