Can we as Catholics attend a relative’s Jewish wedding, who is converting to Jewish?
I assume you mean that your relative is a Catholic who is apostatizing from his or her Christian faith to convert to Judaism in preparation for marrying a Jew. In such a case, so far as I can tell given the conditions you outlined, it is very possible that the marriage will be presumptively invalid. There is also the issue of appearing to accept and/or support the apostasy of a relative from the Christian faith. Under such conditions, I cannot recommend attending such a wedding or participating in any of the festivities surrounding it (e.g., shower, rehearsal dinner, reception, etc.).