What I was thinking is, for example, your founder supermaxes the science skill. Then the founder goes around and obtains DNA samples from the sims whose genes you want for future spouses. Then, when each subsequent heir is born, the founder can use the DNA sample to clone the future spouse. The clone does not appear on the family tree, and so has no familial relationship, but your family can still raise them (like Trip said, akin to an Imaginary Friend).