Should a sunni person participate in the nikkah or the marriage ceremony of a shiah cousine which is already converted from sunni. Should the shiah cousine be congradulate also on his marriage ceremony or what way should be adopted in the light of shariah.

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.

It is permissible for a Muslim to attend the wedding ceremony of his cousin who is an innovator, and the right of this cousin on him is that he should congratulate him unless there is in the wedding something forbidden, like consuming intoxicants, dancing, music, forbidden mixing, and any forbidden innovative practices. However, he is obliged to advise him and call him to the methodology of Ahlus-Sunnah and clarify to him the danger of following the deviated methodologies.

Moreover, if there is a benefit in deserting him, like changing him, then this is permissible. In this case, he should inform him that he is deserting him because of his innovation. For more benefit, please refer to Fataawa 84976 and 86090.