Can i claim a step child as a dependent when filing married but separately?

Yes, you can claim your stepchild as a Qualifying Child dependent (filing as Married Filing Separate) if:

• The child must be related to you. The child can be your son, daughter, stepchild, eligible foster child, brother, sister, half-brother, half-sister, stepbrother, stepsister, adopted child or an offspring of any of them.
• Your child must be under age 19 or, if a full-time student, under age 24. There is no age limit if your child is permanently and totally disabled.
• Your child must live with you for more than half the year, but several exceptions apply (ie. away at school).
• The child cannot provide more than half of his/her support.
• You must be the only person claiming the child
• The child must be a US citizen, resident alien, national, or resident of Mexico or Canada.
• The child cannot file a joint return with his or her spouse.

See Rules For Claiming Dependents

if my husband has the right to file the dependency credit on his daughter, court documents state he gets to claim her every year, can he allow me to claim her if we file married filing separate? she does spend a little more time with her mom during the year, but her mom can't claim her due to the court order. So can he allow me to claim her instead of himself?

Me an my ex wife are separated but her three children reside in my home, am I able to carry them on my return because I got audited an the tax place I went through said I was allowed to carry them after they told me I could 

You can claim the stepchildren on your return as qualifying relatives as long as the situation meets the eligibility qualifications.

Yes, you can claim your stepchild as a Qualifying Child dependent (filing as Married Filing Separate) if:

• The child must be related to you. The child can be your son, daughter, stepchild, eligible foster child, brother, sister, half-brother, half-sister, stepbrother, stepsister, adopted child or an offspring of any of them.
• Your child must be under age 19 or, if a full-time student, under age 24. There is no age limit if your child is permanently and totally disabled.
• Your child must live with you for more than half the year, but several exceptions apply (ie. away at school).
• The child cannot provide more than half of his/her support.
• You must be the only person claiming the child
• The child must be a US citizen, resident alien, national, or resident of Mexico or Canada.
• The child cannot file a joint return with his or her spouse.

Here's a checklist for determining whether your mom (or other relative) qualifies.

• Do they live with you? Your relative must live at your residence all year or be on the list of “relatives who do not live with you” in Publication 501. About 30 types of relatives are on this list.
• Do they make less than $4,300 in 2020 or 2021? Your relative can't have a gross income of more than $4,300 in 2020 or 2021 and be claimed by you as a dependent.
• Do you financially support them? You must provide more than half of your relative’s total support each year.
• Are you the only person claiming them? This means you can’t claim the same person twice, once as a qualifying relative and again as a qualifying child. It also means you can’t claim a relative—say a cousin—if someone else, such as his parents, also claim him.

 Typically, the person with whom the children live with over half the year will be able to claim the dependents on their tax return. But there may be a separate legal agreement stipulating the other parent may claim the children as dependents.

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