Without reviewing the Trust document I can not say what your rights are. But if the stamp collection is part of the Trust it would be highly unusual for either you or your sister to be able to sell it without the Trustee's permission.
But, even if you could do so, it is a truly terrible idea on many levels: once you have given your rights to the stamp collection to your sister, and have acknowledged that it was a gift, she is under no obligation to give you any portion of the proceeds when she sells it. In addition, by making a gift of your interest, which appears to be in excess of $200,000.00 you potentially incur gift tax liability, then assuming your sister does give you 50% of the proceeds when she sellls the collection, she also incurs potential gift tax liability. There are many other good reasons not to proceed with this plan as well.
Answered on May 16th, 2013 at 11:31 AM