To sell the entire house in a voluntary transaction with a particular buyer, everyone who is in title (or someone with authority to act on their behalf, such as a power of attorney agent or guardian) must execute the deed.
Transfers Of Partial Undivided Interests
If the house has two or more co-owners, who own undivided percentages of the whole, either as tenants-in-common, or as joint tenants, one co-owner can sell their undivided partial ownership interest, but that leaves the buyer as a co-owner with the co-owners who did not sell their interest in the house.
As a practical matter, the commercial reality is that no arms-length third-party buyer and no ordinary mortgage lender would agree to a transaction in which the buyer did not receive 100% ownership of the house.
But, in family transactions, it wouldn't be too unusual for one family member to give their interest in the house to, for example, a spouse or their children, leaving the gift recipient as a co-owner with the other co-owners of the property (who are usually family members or members of two or three families who are close friends).
This does not vary meaningfully from one U.S. state to another, except that one spouse in property held by spouses as "tenants-by-entirety" (a form of ownership only available in a few states in the Northeast), cannot transfer a partial co-ownership of the house to a third-party.
Sales Of Homes In One Name By Married People
There is variation between states on the rights of a single person in title to real estate who is married, in a community property state, to sell the real estate without a spouse's signature.
Pennsylvania is not a community property state, so this consideration does not apply there.
Historically, a spouse's signature was also required in some states (e.g. New York State) to waive "dower rights" in a residence sold by their spouse which was solely in their name, but I am not aware of any U.S. state where this practice continues in force that is not a community property state.
The other exception is that when a house is co-owned, any owner may bring a lawsuit at any time, called a partition action, forcing the sale of the property (often at auction) if the parties can't reach a mutual agreement on how to sell the property and distribute the proceeds, or how to buy out some co-owners. But a partition action can't be used to force a sale to a particular buyer in a particular transaction. It just forces the property to be put up for sale, with the first person to pay an agreed listing price, or the highest bidder, buying the property.
Partition actions are not allowed when the co-owners are separated in time rather than being concurrent co-owners of the house.
So, for example, if one person has a life estate in the house, and another person has a remainder interest in the house, both must sign off the sell the house and a partition action is not available to force a sale in most jurisdictions.
This answer is based upon U.S. law. The law in England regarding co-ownership, despite being a common law country and the source of U.S. law regarding real estate (outside community property states), is materially different from the law in the U.S.