When you say PA do you mean Pennsylvania? If that's the case, you really ought to post this question for Avvo lawyers who are licensed in Pennsylvania. Am I understanding your question correctly?
In California, it would depend on what the time share agreement is. Usually it doesn't differentiate between the owners of the actual time share who share ownership unless there is another agreement. You could ask for declaratory relief as to your actual rights and have a judge decide. You could ask for a partition, although what would you really gain from it? What you really want is the additional two months that the other partner has been using the property.
Is there any agreement in writing? I couldn't give you a more comprehensive answer without reviewing your documents. I would suggest that you speak with a lawyer licensed in the state where the propery is located.
Where was the agreement entered into?
How did they acquire the property? Did they purchase it or inherit it? Or was it otherwise gifted to them?