小 D's story -- nobody to marry

My older son was in kindergardon several years ago. One day, he sat with his older sister in backseat in the car. They argued with each other very often, but they talked along very well that day. Suddenly my son changed topics and told her sister:"I will marry you when I grow up."
"We are a family,  you can not marry me.", said my daughter.
" ok, I am going to marry mom."
" you can not marry mom; mom was already married to dad."
" ok, I am going to marry Dan's mom."
Dan is my neighbor‘s kid and his mom is very nice to my boys.

" Dan's mom already was married to his dad",again, reminded my daughter.
"Then, who should I marry?", said my older son sadly.