Below is the Texas Penal Code regarding "incest" Based on the Penal Code, it would not be illegal for you to marry the widow of your Nephew because there is no blood relation.
Sec. 25.02. PROHIBITED SEXUAL CONDUCT. (a) A person commits an offense if the person engages in sexual intercourse or deviate sexual intercourse with another person the actor knows to be, without regard to legitimacy:
(1) the actor's ancestor or descendant by blood or adoption;
(2) the actor's current or former stepchild or stepparent;
(3) the actor's parent's brother or sister of the whole or half blood;
(4) the actor's brother or sister of the whole or half blood or by adoption;
(5) the children of the actor's brother or sister of the whole or half blood or by adoption; or
(6) the son or daughter of the actor's aunt or uncle of the whole or half blood or by adoption.
(b) For purposes of this section:
(1) "Deviate sexual intercourse" means any contact between the genitals of one person and the mouth or anus of another person with intent to arouse or gratify the sexual desire of any person.
(2) "Sexual intercourse" means any penetration of the female sex organ by the male sex organ.
(c) An offense under this section is a felony of the third degree, unless the offense is committed under Subsection (a)(1), in which event the offense is a felony of the second degree.