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Under Texas law (and most other states), it is considered incest if the siblings are biologically related or adopted.
Here's the statute:
Tex. Penal Code § 25.02. Prohibited Sexual Conduct
(a) An individual commits an offense if he engages in sexual intercourse or deviate sexual intercourse with a person he knows to be, without regard to legitimacy:
(1) his ancestor or descendant by blood or adoption;
(2) his stepchild or stepparent, while the marriage creating that relationship exists;
(3) his parent's brother or sister of the whole or half blood;
(4) his brother or sister of the whole or half blood or by adoption; or
(5) the children of his brother or sister 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.
Thus, if adoptive siblings were to marry, the marriage would be void and illegal. Thus, this is not possible to be done legally under Texas law.