Where did the blue twins come from???

Completely out of nowhere. Ok my current sim is about 7th generation from my SIM self on long life span. She and her twin sister are the oldest of 5 kids. Their father is Asian and their mother's mother was a green but beautiful alien born from abduction along with her twin brother. Yes great Grandpa got abducted and had green twin alien babies. One was Lexi. Lexi had Antoinette. Antoinette married Bo Chin and all five were green but weren't aliens like their mother and grandmother. Tiara, Alisa, Cecilia, Kameron and Vincent. The first 4 are married with children now. Vincent is still a teen. Alisa has boy/girl twins, one green and one blue. Blue? I thought Ok, a fluke. Won't happen again. Then Kameron got married and had boy/girl twins, one green and one blue. Tiara and Cecilia have green and human skin babies but no blue ones so far. Not blue sims in their ancestry that I have found. Perhaps I should take a closer look at their spouses Ancestry but most are deceased so I will have to check for other pollination techs.