Q: Is it a sin for a non-Jew to be invited to chant an aliyah during a Bar Mitzvah?

A: It's not a sin, but it is not usually permitted because the words of the prayer suggest that the one saying the prayer is Jewish. If the person saying the prayer is not Jewish, the actual saying of the prayer would be what we call "saying a prayer in vain" - in other words, saying a prayer that is not true or accurate in some way. This is not permitted by Jewish law.

That being said, some synagogues make exceptions anyway for close relatives who are not Jewish, or find other ways for non-Jewish friends and relatives to participate in the service. However, the way in which this is handled varies from one congregation to another and is usually regulated by the rabbi or the ritual committe or both.