Every state has its own probate laws. Even then, those probate laws change in applicability according to whatever will was left. In most cases, if there is no will or stipulation that someone's assets go to a particular person or organization, then the current spouse is usually the heir. If there is no spouse, then it goes to children and whatever succession of heirs dictated by state law. If your father or father-in-law had a legitimate will that his spouse did not get anything and everything went to his beloved pet ferret, then the ferret would get everything and you and the rest of your relatives would get nothing. In D'Andra's case, she has stated multiple times there were two wills: one that left everything to her and one that left everything to her brother. The legal battle was over which will was the legit one, or what was the father's last wish of who was to inherit his money?
Where D'Andra had her claim was that Mama Dee said an episode or 2 ago that she and D'Andra's father agreed that he wouldn't pay child support with the stipulation that he would leave everything to D'Andra when he died. That's the will that Mama Dee and D'Andra have. Apparently, and unsurprisingly, he had a change of heart after marrying stepmom and having a son, and he drew up a second will stipulating that his son would inherit everything. That's the will stepmom had. There must have been some sort of language in either will, or both wills, that didn't negate the other full stop, hence the expensive legal battle.
It sounded out of character for Mama Dee to agree to no child support if D'Andra got everything when her father died, because Mama Dee has show herself to be extremely selfish and greedy, and it surprised me she would agree to something like this that had a higher chance of her not getting any money.
Here's my theory on what went down; I'm purely speculating here. I suspect Mama Dee got caught stepping out on D'Andra's father and he had hard proof that would guarantee him not having to pay alimony to Mama Dee. Mama Dee knew this, but she had her ace card of D'Andra, in which she knew she could get him to pay through the nose in child support. Daddy Dee didn't want to pay child support, because he knew Mama Dee would spend the money on herself, not on D'Andra, so he probably initially tried to set up a trust fund where he would make the child support payments so that and D'Andra could access the trust fund when she turned 18. Mama Dee probably didn't like that, because then she doesn't get to control the money, which is Daddy Dee's whole point. Mama Dee probably fought that or the law or judge wouldn't agree to it, or something. For whatever reason, they reached the agreement (more likely a compromise after a lot of legal battling) that Daddy Dee didn't have to pay child support if he left everything to D'Andra, and probably had to draw up said will immediately and give a certified original to Mama Dee. I also suspect that Mama Dee had already locked down Simmonds by then and knew she would have his money to afford her the lifestyle she wanted to be accustomed to, which is the only reason she agreed to the will compromise. It was never about providing for D'Andra; Mama Dee wants money, and lots of it, and SHE wants to be the ONLY one in control of it. Remember: she is the trustee of the trust fund that Simmonds left to D'Andra, meaning D'Andra has to go to Mama Dee anytime she wants to access the trust.
Mama Dee has shown herself to be a very selfish, vindictive, controlling and extremely greedy woman, who has no qualms about hurting her daughter if she feels like it, time and time again on the show. And, that's just what we see on the show! I still remember D'Andra saying in her first season how she had a happy and successful life in D.C. when Mama Dee begged her to come back to Dallas and run the company, because Mama Dee was tired and wanted to retire. D'Andra finally caved and completely uprooted her life, and then Mama Dee handed the company over...about 12 years later. Mama Dee didn't want to retire; she wanted to control D'Andra. It would serve D'Andra well if she got therapy and cut Mama Dee out of her life. I don't think that'll ever happen, because I think Mama Dee has manipulated D'Andra so much over the years that D'Andra probably thinks she can't function without her. While I'm not necessarily a fan of D'Andra, I absolutely HATE how her mother treats her. I can only imagine the mommy dearest she was during D'Andra's childhood and wish D'Andra would tell her to fuck off and completely walk out of Dee's life. That would be the only way to "get to" Dee, and it would probably be the only way to earn her respect. I absolutely despise Mama Dee.