Is it possible to change your cousin's gender with this or do you have to restart the whole game? I'd like to fight against Jill instead of Jack but I'm not sure if just changing their physical features will change how the game views them.
Yes. Jill Steele has the following stats:
* Hip and butt ratings of 6
* Breast size of 4
* 75 Femininity
* A single human vagina with 1 Looseness, 1 Wetness, 1 clit and a hymen
* No male genitals (i.e. cock/balls)
* Lacks vaginal virginity, but retains penile and anal virginities
While Jack Steele has:
* Hip and butt ratings of 2
* Breast size of 0
* 25 Femininity
* A single 12" human cock with a thickness ratio of 1.5 and 2 balls of size 2
* No female genitals (i.e. vagina)
* Lacks both penile and anal virginities, but retains vaginal virginity
All other stats for both of them are the same. Just change their name and modify their stats to fit the above, and you should be good to go. I'm not sure how this will interact with
the lynx-boy/girl appearance they get as a disguise for the party on Dhaal; however, I pulled up a file from after that event and found that their body parts were still all human, so I doubt save editing at this point will affect anything.