Hope you don't mind me adding my 2cents? AFAB here (and also did a fair bit of ballet)
I always found crossing one side easier than crossing the other side. I have to assume it's partly muscle tightness on that side or relative muscle strength in holding the position.
I also found it much more taxing to cross my legs and hold the position when I was more overweight. It does make a difference for me.
I can't speak much for the specifics of flexibility require for crossing your legs in a sitting position, not something I've looked into!
But in general, basic flexibility is more about muscle relaxation than about ligament laxity (unless you are looking for ballet-style hyperextension) Most people should be able to achieve the splits, but it's best done by muscle release and learning which bits tighten up in you and need work. From head to toe! Spasms and tightness anywhere down the chain can pinch nerves (so they cannot slide when you stretch) and prevent flexibility.
Hips I'm not so sure about. I know that there are definite limitations in turn-out due to bone structure of the joint. Not sure that turn out is relevant to leg-crossing though. As was said, there are skeletal differences between men and women, the relevant one likely being the Q angle of hips. Women tend to have a bigger Q angle than men so the thighs point more across/in to begin with.
Hormones do affect ligament laxity. I have hypermobility syndrome/EDS and I can tell when my period is due because of the increase instability, subluxes and general chaos and pain! Certain female hormones make the ligaments more stretchy (partly why pregnant women sometimes use pelvic supports because the ligaments loosen for childbirth and pelvis kind of pulls apart).
But be careful what you wish for! It's better to become more flexible through muscle release than ligament stretching - lax ligaments lead to instability, dislocations, sprains and a host of other problems.
Off the top of my head I'd imagine that the best muscles to stretch for crossing your legs will be IT-band related, abductors and piriformis area (though lots of little bits around there than can tighten up, tennis ball releases can help you find them) Maybe some strengthening of the inner thigh/adductors.
Also finding the precise position that is most comfortable for you. How high up the femur you try to start to cross, how you angle the underneath leg and shin. or cheating a bit by resting the back of the top knee over the lower one.
FWIW I think a lot of cis-women feel uncomfortable after a while, and need to shift position.
Women didn't start sitting with crossed legs because it was comfortable or natural, but because it was more "seemly".
Hope you find something that works to achieve what you want! :-)
