Kerstin is the name of Doug's ex-wife. Doug is Kamala Harris' husband. The three have a very interesting relationship. Doug cheated on Kerstin and that led to their divorce, but they continued being friends and co-parented to perfection.
When Doug and Kamala got married, the three formed a very functional blended family that saw the two women become fast friends, even attending the children's school activities together. If ever there was one, this is a happily ever after thing.