A deceased female relative was never married to the person who is her partner. However, they had children and lived together many years.
If you click on the Edit button at the right of the couple box in Family Members, in Relationship Events you can click on Add Event.
In the top line drop down there are several possiblities to describe this relationship including 'Lived together'.
See this article in the Help Centre: https://www.familysearch.org/help/helpcenter/article/how-do-i-add-or-change-marriage-information-in-family-tree
Hope this helps.
You juste enter them as a couple without date and marriage place and it's work.
Hope it's willbe helpfull