The GIRM (paragraph #74) says that the norms for singing at the Offertory are the same as at the Entrance. Those are in paragraph 48. One of the four options is "another liturgical chant that is suited to the sacred action, the day, or the time of year...."

Since the day in question is a memorial of our Lady, it is therefore quite fitting to sing an offertory in Her honor.

While the Sister wasn't taking those provisions into account, God bless her for her thoughtful interest in correct liturgical observance!

and that it must be about the bread and wine that is being offered

This is indeed incorrect. I didn't a full search on the offertory texts, but bread is only mentioned once (Portas caeli), and wine is never mentioned in any offertory. There is only one instance where an offering is mentioned (Sanctificavit). The common themes of the offertory are: following the commandments of the Lord, humble and trustful supplication to the Lord, prayer for the assistance of the Lord.

This fits with the purpose of the ritual that the offertory accompanies: by offering ourselves (symbolised in the bread and whine) and surrendering ourselves to God's will, we are able to enter the sacrificial death of Christ. Mary is the prime example of someone who surrendered to God's will (Behold, I am the handmaid of the Lord. May it be done to me according to your word), and she more than once appears in the offertory texts.

"diocesan liturgist fad session"

Yes, I've been getting an earful of this kind of thing lately when talking about music for the new Missal.

The loss of the sung propers led to a kind of speculative creativity that has been completely unhinged from the Mass and history.

Had that problem, young priest turned to pastor and said:

"Is she not the Mother of God?"

And if it will make Sister happy, Mary (like Adam and Eve) is "virgin earth" -- in her case, the virgin earth from which the Bread of Life, the True Vine grows. So all sorts of rationales come together in a BVM offertory proper.

Thanks. My search was based on a quick look at the index of Adam Bartlett's Simple English Propers. I believe Sacerdotes Domini is only used in the EF. But even three offertories mentioned the offering of gifts aren't many...

It seems to me that Offertory propers are usually about following up on the previous propers and readings, and not so much about position and action. It really is more like additional readings than about the 4-song model of having a gathering song, an offertory song, a Communion song, and a closing song. Propers are more like breaking into song about the particulars of the day, and thus opening up Scripture wider and wider, entering in deeper and deeper, until you hit the Mystery part of Mass, head on. All Scriptural roads lead to the New Jerusalem, and the angels sing with us as we go up to the Temple and the High Priest and the Victim.

Chonak:
I couldn't find this quote: One of the four options is "another liturgical chant that is suited to the sacred action, the day, or the time of year...." in no. 48. Is that the right paragraph?

Thanks

Except for Solemnities, feasts of the BVM, etc. the Offertory generally follows a long-ago established cycle of singing the psalms in an ascending order* that has since been interrupted by rearrangement of the calendar and reassignment of some of the chants. There's a great chapter on this in Apel's Gregorian Chant. So unlike the the Communion, which is often lifted directly from the Gospel of the day, there is no association between its text and the "themes of the day." However, according to the GIRM, when it is replaced by another song that song should be seasonal or related to the liturgical action.

*edit: Or am I thinking of the Graduals? Someone correct me if I'm wrong, but either way the order is not related to either the old order of readings or the revised Lectionary.