The sixth Blake sister is a married astronaut. While her first name is never mentioned, it can be assumed that it probably starts with "D," since Daphne and the quadruplets all have first names that start with "D."
Just like Daphne and the rest of their sisters, the wedded Blake sister has red hair and purple eyes, and clearly takes after her mother, Nan, in looks.
Rather than wear a traditional wedding dress when she got married, both she and her husband (who's presumably an astronaut like she is) got married in their spacesuits--her groom wore an orange-and-white astronaut suit while she wore an off white-and-pink astronaut suit.
She is so dedicated as an astronaut, that she wore a space suit to her own wedding. She was quite happy having her photo taken.
- Nothing is ever mentioned of the sister outside of the aforementioned appearance, where she was only indirectly referred to.
- While the quadruplets are shown as "underachievers" in the post-Nibiru timeline of the series finale, it's unknown if she was affected by it--it's possible that she wasn't affected by it and still has her successful career as an astronaut, while being married to the other astronaut.