This article gives me a possible motive for Missy's murder:
"According to a search warrant released by law enforcement officials in Midlothian, murdered fitness instructor Missy Bevers was allegedly having marital and financial struggles at the time of her death and had been involved in an intimate relationship external to the marriage.
http://dfw.cbslocal.com/2016/05/05/bevers-was-having-marital-problems-before-murder/
The article is from May, but the fact still remains BB stated in the search warrant whom MB was having an affair with, and from the People article, the mother-in-law is "furious" the investigation has stalled.
I think in that little paragraph from the Search Warrant there is plenty of motive:
◾To keep a secret
◾Revenge
◾Frustration/hate
◾Money/Greed
◾Sex/Jealousy
◾Personal Vendetta
I think it is time for lie detector testing to be done. All I can think is the people named in the SW's had alibis that were tight, but it has been 6 months and some might not have such a tight alibi any more.