If I have a gay brother am I bound to not attend his “commitment” ceremony if I am Catholic?
You are bound to do what is loving and just. You are required to follow your rightly formed conscience. Most importantly, “Ama Deus et fac quod vis” (Love God and do what you will). St. Augustine, Bernard Lonergan and Matthew Fox all say that. If you’ve got those three on the same side, it must be Catholic!
It was only in 1972 that the American Psychological Association decided that homosexual persons were no longer to be considered mentally ill. Society has changed a great deal in understanding of and attitudes toward homosexual persons in recent decades. The church teaches that homosexuals “must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided” (CCC #2358).
Moreover, you will be “losing” your brother if you don’t go to the ceremony and do damage to a relationship that may be impossible to repair. You need not agree with his lifestyle choice to attend the ceremony.