It looks like your stepson and daughter meet the qualifications to be a qualifying child (see details below) for both you and your wife but not for someone else. You could file a paper return and try to claim the children but you will need their social security numbers and be prepared to have proof that you had a right to claim them. The IRS will use the Tie-Breaker Rules if a child meets the rules to be a qualifying child of more than one person.
Tests To Be a
Qualifying Child Tests
1. The
child must be your son, daughter,
stepchild, foster child, brother, sister, half brother, half sister,
stepbrother, stepsister, or a descendant of any of them.
2. The
child must be (a) under age 19 at the end of the year and younger than you (or
your spouse, if filing jointly), (b) under age 24 at the end of the year, a
student, and younger than you (or your spouse, if filing jointly), or (c) any
age if permanently and totally disabled.
3. The child must have lived with you for more
than half of the year (your daughter is considered to have lived with you since
the beginning of the year because she was born in 2018).
4. The
child must not have provided more than half of his or her own support for the
year.
5. The
child must not be filing a joint return for the year (unless that return is
filed only to get a refund of income tax withheld or estimated tax paid).
