colorado
The couple separated and she now is trying to threaten him with having
to keep the kids 50% of the time and pay child support and they're not
even really his children.
Can she do this?
Yes. Absolutely, although actual parenting time orders have to be entered by a court in the best interests of the child, and 50% is not a magic number in that regard.
More generally, children are "really your children" even if you aren't biologically related to them under Colorado law. Someone does not have to be biologically related to a child to be their legal father.
In Colorado, a man is presumed by law to be the father if the man consents to having his name placed on the birth certificate of a child of his wife after the children is born, even if the marriage took place after the child was born.
(1) A man is presumed to be the natural father of a child if: . . .
(c) After the child's birth, he and the child's natural mother have
married, or attempted to marry, each other by a marriage solemnized in
apparent compliance with law, although the attempted marriage is or
could be declared invalid, and: . . . II. With his consent, he is
named as the child's father on the child's birth certificate[.]
Colo. Rev. Statutes § 19-4-105.
In Colorado, and most other jurisdictions there is a statute of limitations from the birth of the child to contest paternity established by a birth certificate. In Colorado that is five years. In some jurisdictions it can be as little as two or three years.
(1) A child, his or her natural mother, or a man presumed to be his or
her father pursuant to section 19-4-105 (1)(a), (1)(b), or (1)(c) or
the state, the state department of human services, or a county
department of human or social services, pursuant to article 13 or 13.5
of title 26 or article 5 of title 14 may bring an action: . . .
For the purpose of declaring the nonexistence of the father and child relationship presumed under section 19-4-105 (1)(a), (1)(b), or
(1)(c) only if the action is brought within a reasonable time after
obtaining knowledge of relevant facts but in no event later than five
years after the child's birth. After the presumption has been
rebutted, paternity of the child by another man may be determined in
the same action, if he has been made a party.
Colo. Rev. Statutes § 19-4-107.
If the conditions for the five year statute of limitations applies, other statutes of limitations that would otherwise seemingly also apply cannot be utilized. See e.g., People ex rel. S.L.H., 736 P.2d at 1228; see also M.R.D. by P.D. v. F.M., 805 P. 2d 1200, 1201-03 (Colo. App. 1991).
If the children are six years old, the statute of limitations to disestablish the paternity of the father listed on the birth certificate shortly after the children were born has passed and the presumption of paternity is conclusive and cannot be rebutted with even conclusive evidence of a lack of a biological relationship between the father and the child.
The statute of limitations is longer in cases in which no man has been established to be, or is presumed to be, the father of the child, or when the basis of the presumption of paternity one of several grounds which are each different than the one in this case. But, that isn't the situation in this question.
Also, if the man agreed knowing the facts, he may be estopped from changing his position later on even if he contests paternity in a time frame consistent with the statute of limitations. See generally, Colo. Rev. Statutes § 19-4-105(2).
Furthermore, only certain people are allowed to bring suit to raise the issue that the husband of the mother is not the father - only the mother, the legally recognized father-husband, or the child (through a guardian ad litem). In the fact pattern of this question, "the other man" or his relatives are not allowed to bring a lawsuit to have the paternity of "the other man" (a provision of the law that the U.S. Supreme Court has held to be constitutional in a case arising in another state). The government also lacks standing to bring a lawsuit in it own right to declare that the father-husband is not the child's parent.
Child support is owed by the legally recognized father of a child regardless of the existence or absence of a biological relationship to the child, unless at least such time as a court determines that this person is not the legal father of a child. And, often due to a conclusive presumption or a statute of limitations the legal father's paternity can't be disputed at all.
The substantive and most of the procedural details are spelled out in the Colorado Uniform Parentage Act.