That's not really insist, Sims only know so many people. I don't think they count cusins as true bloodline. Even if your family tree is huge.
In some countries marrying and have kids with your cusins unless they are your 5th cusin is bad genetic practice for your family tree. Insist is more about marrying in the purity of your bloodline, for example daughter marries sister of blood line relative, however, daughter marrying cusin is not really insist just bad behavior for enriching the gene pool.