Non-Belief In The Real Presence

[Ignatius]

My sister, who was basically a non-practicing Catholic recently returned to the church last year at my encouragement. She came over for Thanksgiving yesterday and we got into a discussion about the Eucharist. My sister surprised me when she said she had doubts about the Real Presence and that communion was basically symbolic (like the Protestants believe). Needless to say, I was shocked, so my question is, now that she goes to Mass every week which includes receiving communion, is she committing sin by receiving because she doesn’t believe in the Real Presence?

Hopefully the following Scriptures will help her to see the seriousness of this.

1 Corinthians 11:27-29
Partaking of the Supper Unworthily

27 Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. 28 Let a man examine himself, and so eat of the bread and drink of the cup. 29 For any one who eats and drinks without discerning the body eats and drinks judgment upon himself.