My sister is in a lesbian relationship and has just informed me that she is going to marry her partner next July. She asked if I would join her to celebrate this event. As a devout Catholic, should I pay respect and show up to the ceremony? (She knows that I don't approve of this type of relationship.)
Then she shouldn't be asking you to "celebrate" it with her. To do so could be a means of seeking to undermine your commitment to your principles. You should, of course, be kind to your sister and be available to her should she need you. But I cannot recommend attending her "wedding" under any circumstances. Marriage is a sacrament; the abuse of a sacrament is a sacrilege. That means that any attempt to ape marriage is an objective sacrilege. Catholics have a duty not to participate in sacrilege.
What I can recommend is saying to your sister, as gently as possible, "You know where I stand on this matter. Please do not ask me or expect me to violate my conscience."