Live simply, so that others may simply live.
Attributed to St. Elizabeth Ann Seton, sometimes changing the word “others” to “all.”
She doesn’t have a Wikiquote page at all.
A general Google search shows that it’s also widely attributed to Gandhi, but in my (admittedly non-thorough) search of this aspect of the question, never with a reliable source. He does have a Wikiquote page, but this quotation is not on it nor on the associated discussion page, which is where unsourced quotations usually go.
Back to Mother Seton … the general Google search does show that it’s often sourced to a “speech given in Baltimore,” unfortunately with no more details than that.
The earliest Google Books appearance is from 1980, in The American Baptist Woman (I am not making this up), vols. 24-26, p. 30, which says, “Sister Elizabeth Seton is credited with saying …,” followed by the quotation. Unfortunately, if the author said where she is credited with saying it, it’s not visible in the snippet that Google Books will show me.
DePaul University’s library has an online collection of Mother Seton’s works. I searched for the word “simply”, which returned hits in her Collected Writings, vol. 1 and vol. 3a. Her time in Baltimore is covered in vol. 2, so I downloaded that too. I can’t find the quotation in any of these places.
At this point the best I can say is that I am not convinced that she didn’t say it, in large part because the “speech in Baltimore” reference makes me think that perhaps someone once found it somewhere.