My sister and I are buying a house, but since she is still married with her soon to be ex-husband, the mortgage company is saying that he is required by law to sign the deed or. he could sign a release form. but he doesnot want to sign the deed for the new house, and we can not buy the house without his signature. so what our options are, and if it is possible to buy the house without the soon to be ex-husband's signature? Thank you!

try to create a trust or limited liability partnership between you two and you purchase it alone and transfer after the divorce.
you can get your mortgage company to stipulate that you will be allowed to transfer an assumption to your sister or what about if your sister co-signs the loan.