Is drinking the water from washing one's mother's hands and feet, then performing ablution with it, considered excessive devotion (ghuluw) and a forbidden form of seeking blessings (tabarruk)?

It is not legislated to seek blessings from the relics of anyone other than the Prophet (peace be upon him), neither from their water, hair, nor anything else; because it has not been reported. The Companions sought blessings from the Prophet’s relics (peace be upon him), but it was not reported that they sought blessings from the relics of Abu Bakr or Umar (may Allah be pleased with them). This constitutes their consensus on abandoning such practices.

Seeking blessings from righteous people is divided into two categories: seeking blessings from their supplications, which is permissible with conditions. The other is seeking blessings from their relics (such as their clothes and belongings), which is not permissible and is an innovation.

Accordingly, it is not legislated to seek blessings from a mother's water, nor by drinking what flows from her ablution, and the like. This is an act of exaggeration. Righteousness towards the mother is achieved by obeying her, being kind to her, helping her, and supplicating for her.