Do we know any of this speculation to be fact?:
1. Misty has a cousin from TN who came down to visit.
2. Misty (and maybe Ron) saw said cousin on this visit.
3. Misty stays at home almost all day with Ron's kids (when they are not at school) and she does not have a job of her own.
4. Misty is 17 years old, not married to Ron, and not the mother of either of these children.
5. Misty has some kind of sexual history (perhaps unwanted) with said cousin from TN.
6. Misty may have family in the area (perhaps where said cousin stayed while visiting).
7. Rumors of Misty going on a bender/missing/hi-tail (whatever you want to call it) for 3 days before Haleigh went missing.
8. Alleged argument between Ronald and said cousin over a gun belonging to Ronald.
WHAT IF (i.e., speculation):
Misty, a 17 year old child, is watching Ronald's kids for him all the time. Perhaps she is tired of constantly minding children and when she hears her cousin is coming to town they all plan to hang out. Perhaps, for whatever reason, when Ron meets the cousin they do not like each other. Perhaps Ron tells Misty not to see the cousin anymore and perhaps Misty goes off on a 3 day hang with the cuz.
If there was an argument over Ronald's gun the day before Haleigh's disappearance then I would assume Ronald knew the cousin was in the trailer (or wherever Ron keeps his gun) and that Ronald thought the cousin stole the gun. Now, I don't know if the cousin got his @ss whupped and came back to the trailer later for revenge or what...
If Misty did not like the cousin at all then it would have been easy enough for her to avoid him when he came to town, right? I think we can safely assume Misty was on some sort of friendly relations with the cousin.
So, maybe after the day of the fight Misty knew Ron was not going to let her hang out with the cuz any more. Maybe she waited until Ron went off to work and then she either let him inside the trailer or maybe she thought she put the kids to bed and left.
Past this, I have no clue what happened.