What is the ruling on someone who says in Ramadan: "Since Allah has chained the devils in Ramadan, how is it that so-and-so is still not chained?" And what is the ruling on whoever laughed at or narrated this story? Are they considered disbelievers? And is it permissible to supplicate for their forgiveness and mercy after their death?

What you mentioned does not appear to be mockery of religion or the Sunnah. Rather, it is a joke or an insinuation directed at a person. The expression "tying" (al-rabṭ) does not show disrespect towards the Sunnah, because the meaning of "shackling" (al-taṣfīd) and "chaining" (al-tasalsul) is "tying," and the word "tying" (al-rabṭ) itself appears in the Sunnah. What you fear is mockery of the person, not of religion. The rule concerning declaring someone an unbeliever (takfīr) is that a Muslim is not declared an unbeliever except with certainty. An action or statement that could be interpreted as disbelief or otherwise is interpreted as not being disbelief. Therefore, your family members remain Muslims.