I didn't see it in comparison to the other main characters. Duh!Spoilers hidden
I thought that was pretty well explained in that one of the Hindenburg survivors descendants marries her mother instead of the person she thought was her father. Viola, no sister.
*light bulb shines over my head*
Lucy is the only one of the three time travelers who has had an ancestor affected by one of the changes in the timeline. Thus, her mother never had cancer, and Lucy never had a sister.
I imagine Wyatt and Rufus will have similar changes in their lives resulting from changes made in coming trips to the past.
I imagine Wyatt and Rufus will have similar changes in their lives resulting from changes made in coming trips to the past.