Im needing some advice for my sister who is dead set on trying to figure out a way to try to marry her fiancee who is her uncle. But my sister is 29 yrs old and she was adopted in tx at 17 and ive already told her its impossible to marry your uncle and arent thinking clearly. But shes stubborn though. Please back me up someone, i need advice so i can relay it to her to try and talk her out of this insane idea I need to know what the state of texas law states about this and other advice.
As far as I can tell merely trying to get married to an uncle isn't illegal in the sense of violating Texas criminal law; the marriage is void, so legally its as though it never happened.
However any kind of sexual relationship would be a serious crime. Since your sister is over 18 she would be considered as guilty as her uncle. This applies even if the uncle is part of her adopted family rather than her birth family.
See here for more details.
A marriage of blood uncle and niece is legal in some U.S. states, notably New York, and the trend seems to be moving in that direction.
In this case, it's an "adopted" uncle and niece, and there should be fewer strictures.
So check in your home state, and if that doesn't work, have the marriage take place in a state where it is legal.