First, the property cannot be sold without the signature of each joint tenants on the purchase offer and on the deed. So, if you don't want to sell for $25,000, then don't sign. It's really just that simple.
If the agent is trying to convince you to sell for $25,000, when there is a bona fide $30,000 offer, then that is a breach of loyalty, and you can sue the agent, and complain to the real estate
The other tenant can sell her interest in the property, at which point the joint tenancy would be severed and you would be a tenant in common with whomever the other tenant sells her interest.
You or the other tenant could sue for "partition," which would cause a forced sale of the entire property. But, the lawsuit would cost more than the entire value of the property, so that would be a total waste of money for everyone.
BotXXXXX XXXXXne, just tell the other tenant that you want the highest and best offer available, and if she doesn't agree, then the property will not sell.
Then wait and see what happens. Maybe the other tenant will "cave in" and give you the $30,000.
Hope this helps.