A unity candle is one of those cultural traditions that are not officially part of the wedding liturgy but that the Church generally allows brides and grooms to incorporate into their wedding ceremonies. Other examples of such unofficial pious customs are giving flowers to Mary, exchanging thirteen coins, and draping the couple with a cord. If a couple doesn’t wish to incorporate customs such as these, the Church also does not object to that.
Gently explain to your mother that at Catholic nuptial Masses the primary sign of the couple’s newly-forged unity is not a unity candle (or other unity emblems that have been marketed by the wedding industry); the primary sign of unity is the Eucharist. Receiving the Eucharist together as husband and wife is a rich sign of your marriage in Christ and is also a sign of your unity in Christ with the assembled community gathered to witness your marriage, as those Catholics among your guests are also invited to receive Communion.
