It says on Mattie's death certificate that she never married. I can see that someone named Cross is attached as her husband in the tree, but I can't see his profile. I imagine that what's going on is that Joseph Franklin Cross, who was married to her sister Mollie, has been attached to the wrong sister.
I made a profile for Joseph and attached it to Mattie - if it's a double of the profile I can't view, someone who can should merge them.
On a different note, it looks like Mattie was buried next to Missie, but her birth and death dates are wrong.