I had a similar problem with my grandmother's adoptive family (her parents died when the children were very young). One brother was living with an uncle, but Granny and her other brother were with another family.
I got their MC, and to cut a long story short, it transpired that the lady was Granny's aunt. She had married using her mother's maiden name. Both parties were under 21 when they married, so we guess that there may have been some family disagreement re the marriage, and they covered their tracks in this way.
Funnily enough the name in this case was also Ellen, sometimes recorded as Helen!