They’re secretly married without her mother’s knowledge.Their marriage took place with an Imam the groom parents, the bride sister and the brother who become the Wali cause their father is already in peace.This situation happens to happened cause the bride’s mom demand to large of dowry and mahr which she can’t be negotiated due to her culture and I don’t know . Is it valid?
The mother’s approval is not mandatory for the nikah to be valid. What is mandatory is that the girl’s guardian gives his approval and consent. If the father is not alive then her brother, or paternal grandfather, then her paternal uncles must act as her wali, without which the marriage is invalid.