Assalaamu alaykum. I have a desperate question; I fear that my sister may have fallen into major disbelief. We were discussing Islam with a group of people, when one of them said, " Muhammad, sallallahu 'alayhi wa sallam, is my role model, to which my sister replied "Why do you not go marry a six-year-old?" I know that what she said was certainly haram, but does it reach the level of major disbelief?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

What your sister said after the statement of the other person does not make her fall into disbelief unless she meant to mock the Shariah or mock the Prophet as this is disbelief without any doubt.

Allah says (what means): {And if you ask them, they will surely say, “We were only conversing and playing.” Say, “Is it Allah and His verses and His Messenger that you were mocking? Make no excuse; you have disbelieved after your belief. If We pardon one faction of you - We will punish another faction because they were criminals.”} [Quran 9:65-66]

We say that this is disbelief in the general sense, but accusing a specific person of disbelief depends on the availability of some conditions and the absence of some impediments, as stated by the scholars.

For more benefit in this regard, please refer to fatwas 8106, and 87963.

Allah knows best.