Is it permissible to speak ill of a Muslim female relative who celebrates Christmas?

Celebrating Christmas is an innovation (bid'ah). Those who engage in it should be advised with wisdom, gentleness, and compassion. Speaking ill of them constitutes backbiting (gheebah), which is a forbidden major sin, unless the woman is openly defiant in her sin, or if there is a legitimate شرعية benefit in her backbiting, such as warning Muslims. In such cases, it is permissible with the intention of advising and warning, not out of malice.