Assalamu Allaykum, is it permissible for one to celebrate the 3 days Eid-ul-Adha by staying over the residence of one's sister who is a Muslim married to a non believer which is not allowed. What is your advise in keep the relation under these circumstances.

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

There is no doubt that that Muslim sister has committed a forbidden act which is unlawful according to the consensus of all the Muslims; for more benefit, please refer to Fatwa 81220. Therefore, it is an obligation to advise her and frighten her from Allaah and remind her that her life with that man is adultery and not marriage and that she must repent to Allaah.

If she is not deterred, she should be deserted if it is hoped that this will be beneficial for her. For more benefit on the ruling of deserting a dissolute person, please refer to Fatwa 84976.

Nonetheless, it is permissible to keep ties with her because it is permissible to keep ties with one’s non-Muslim sister let alone if she is a Muslim, especially if it is hoped that keeping ties with her will affect her. Asmaa' Bint Abi Bakr said: “My mother came to visit me while she was a polytheist at the time of the Prophet so I asked him [the Prophet]: “My mother came wishing that I will be kind to her as her daughter, shall I keep ties with her? He said: “Yes, be dutiful to your mother.” [Al-Bukhari and Muslim]

Allaah Knows best.