You're going to need a lawyer. Since the title doesn't specify the ownership percentages, your in-laws own 50% of the house.
Tenants in common are considered co-owners of a property. This could be two young adults, a group of seniors, family members, or anyone for that matter. Each co-owner has a specific share of the property and decision-making power that can be split any way they want (such as 50/50, 60/40, or 50/30/20).
It’s important, however, that the percentage each owner holds is specified on the title. If it isn’t, every tenant in common will be entitled to an equal share of the house.