Pioneer
She came across the plains to Utah in 1859 at the age of 35, but in an unidentified company.
She had 5 children, ranging from the ages of 1 and 12.
Information from the Mormon Pioneer Overland Travel Index, with links to the person:
Rees, Sarah Margaret Ellis (35)
Rees, Edmund (45)
Rees, David (12)
Rees, Edmund (1)
Rees, Ellis (9)
Rees, Mary (6)
Rees, Thomas (3)
Here is the source. Type in “Rees” as the surname, and 1959 as the year.