Did some digging and I found some REALLY old pictures from 2015 of when the legacy first started. These are the founders.
Here they are in their old age. The female is an alien, but her husband is completely human.
https://i.gyazo.com/84ab07cc765bbf865dcc0d23a2e97c59.jpg
Her husband was actually abducted by aliens and impregnated as an elder. I think he gave the baby up for "adotion" (i made a family and dragged the baby to their household) since they were way too old to be taking care of babies at that point.
https://i.gyazo.com/769c98ce8d67cc065d2efaf3b9a0fcfa.jpg
They did eventually pass and I buried them together. They've been dead for quite a while now in that save.
https://i.gyazo.com/ef871498cec9cdf2d926aaaf4e50ce90.jpg
This isn't even close to the full legacy, I think it got up to 10, the longest in any sims game i've ever done. These are just their kids. The rules of that legacy stated that the oldest born with the most alien dna would become heir. So if a half alien was born and THEN a full alien, the full alien would still be heir even though they're younger.
https://i.gyazo.com/25636cebe523294f7fa0f25ea24dae2f.png