What is the ruling on shunning a maternal aunt due to her disobedience and rebellion against the Sharia, such as her marriage without a valid Islamic contract and her removal of the hijab, and is shunning permissible until she repents from these actions after being advised?

The maternal aunt holds a special status in Islamic law; she is like a mother, and maintaining ties with her is obligatory. Ibn Umar (may Allah be pleased with them both) narrated that the Prophet (peace and blessings of Allah be upon him) said to a man who had committed a grave sin: "Do you have a maternal aunt?" He replied, "Yes." He said, "Then be dutiful to her."

You must maintain ties with your maternal aunt, enjoin good upon her, and forbid evil. It is permissible to abandon her if abandonment leads to her rectification. Marriage has pillars, and the administrative contract is a means of documentation, not a condition for the validity of the marriage itself.