I see why Serie forced the two to become a pair, they are both looking for something. Land looking for a place to bury his family's remains and Ubel, with how it is impied, is looking for her family. Sadly, Land doesn't know he already got the reins on her but I'm pretty sure Ubel knows with how perceptive she is.
Let's be real with Fern thou, she follows Frieren as she enjoys watching her look and collect magic. She will do anything to not hinder in her endeavors and knowing how much hard labor and time is required in doing the laundry before the industrial era is, it's quite understandable why she asked for it.