When two (or more) people own a property as joint tenants, there is a right of survivorship. This means that each of them owns the entire property, just as both would own every dollar in a joint bank account. There is no "half" in a jointly owned property. Each owns 100%. Therefore, there is no "half" to sell.
This seems to be a really tough concept for people. I'm constantly asked about this issue on my blog and at pretty much every seminar I present. One person argued with me for about ten minutes about the fact that if two people owned a house it HAD to be half and half. He had no legal reason to believe this, but that didn't stop him from being absolutely sure that he'd know this better than I would.
The whole idea behind a joint ownership is to leave the entire property to the other person when one dies. For example, if a husband and wife own their home jointly and one of them dies, the other still has the house. It's a "last man standing" concept. It doesn't really work for a number of siblings owning a house.
If two or more people want to own "halves" or "parts" of a house, they should hold that property as tenants in common. In that arrangement, each has a piece of the house that he or she can sell or leave to someone in their Will.
This brings me to the second point.
About 90% of the questions I get about how to get out of a joint tenancy come from people who own something with their siblings and the arrangement isn't working. It's almost always because a parent left the house to ALL of the children equally. Parents need to be realistic about the million or so ways this arrangement can go wrong. More than one child wants to live there, and nobody can agree. One moves in but won't move out and won't pay rent or expenses. The roof needs repairs but each wants the others to pay for it. One rents it out without the permission of the others. One causes damage and won't pay for repairs. Two want to sell but the third won't sign. And on and on and on.
Why wouldn't the parents choose one child to take the house as part of his share? Or direct that the house be sold and the proceeds split? This is truly something done without thought that leads to so many fights and headaches, I can't believe anyone does this anymore. Parents - please stop doing this to your kids!