My daughter's marriage theory

When my daughter was 4 years old, she had a theory that "Mommy marries daddy,  I marry Gege(older brother)". First I didn't think it makes any sense, later I realized that she meant only relatives can marry each other. So she propagated this theory to our friends' family which has a boy and a girl. Then I started to teasing her, how can Glenda and Wendy get married (A friend has two girls)? Or how can Jonathan marry Jalon (A friend has two boys.)    My son who is 2 years older didn't like marriage proposal very much. Especially when my daughter annoyed him one time, then he claimed " I am so not going to marry you! "