My sister is getting married and she asked me if a non-Catholic can participate in the wedding, like carrying the gifts during the offeratory. I told her that only a Catholic can participate explicitly in the Mass. Is this correct?
I suggest that you recommend to your sister that she consult with her celebrant for guidelines on the participation of non-Catholics in her nuptial liturgy. Certain duties should be restricted to Catholics (e.g., readings, distributing Communion as an EMHC), but others need not be (e.g., ushers, greeters). As for the specific example of carrying the gifts in the offeratory, this is not necessarily restricted to Catholics. The first time I participated in carrying the offeratory gifts was as a non-Christian before I entered RCIA.