OK - found her: that means her birthday was about 1905 and from VT - that helps.
--VT Census says her middle name is Mabel: parents Byron and Bertha : she had these siblings: Ralph N Rockwell, Hazel G Rockwell, Jennie L Rockwell, Leon E Rockwell, William L Rockwell in 1910.
As Norman was born in the 1894: and not listed as a sibling here: there isn't a very close relationship (she wasn't sister to Norman).
As the wiki page said the Rockwells came from much earlier than 1800s to the US, and Glenna's parents are from Canada: I don't immediately see a connection.
Here is a long shot:
There is a Mary Rockwell that was born in the US in about 1812: that lived in the same town as Byron Rockwell (your grandmother's Dad): https://familysearch.org/pal:/MM9.1.1/M422-NK5
It may be possible that if you go far enough back in the US with Norman Rockwell: that line may be linked to this Mary Rockwell. If so, then you are likely related.
Sounds like an interesting mystery...