Quick answer
Custody jurisdiction follows specialized rules; it is not simply whichever parent files first.
Do now
Create a child residence timeline, including temporary absences and who lived with the child. Gather all custody orders and identify pending cases. Ask about initial jurisdiction, continuing jurisdiction, and any emergency exception.
Watch for
A court’s power to grant a divorce or support order does not automatically establish custody jurisdiction. Do not conceal another proceeding or assume six months in a new place ends the old court’s authority.
Official sources
DC Code § 16-4602.01 — Initial child-custody jurisdiction, NY Courts — Starting a Case in Family Court, DC Courts — Family Court Self-Help Center.