It is odd that the change was so long after her accident. Is there anyway for her to be seen by her doctor again? I know you said she is being very hostile and aggressive, and that makes it very difficult to get her to seek help. Maybe you could contact her doctor on your own and ask general questions about whether or not she could suffer symptoms this long after her accident. It is important to clarify what can be caused by her brain injury so you understand where the behavior is coming from.
She may very well be vulnerable to her husband and may not be able to determine what is the truth and what is not. From my understanding, judgment can be affect by a brain injury. Her doctor will be able to answer that better for you
If you can clarify the question with the doctor, and they can rule out a physical cause, then you have a mental health issue here. Do you think she may be willing to see a therapist? If not, then you may just have to wait it out. She is an adult and unless she does something to hurt herself or someone else (or even threaten to), she cannot be forced to seek treatment.
What you can do is be there for her. Hopefully, she will develop insight into her behavior and stop acting out. Until then, you can keep the lines of communication open and let her know you love her and want to be there for her. Keep in contact as much as you are able and as much as you feel is appropriate without crowding her. Send birthday cards, invite her to holiday gatherings, etc. You want her to see you as the first person she will turn to if she does want help.
If you can, stay aware of any possible abuse going on in her relationship. If she is being abused, you may be able to help her by offering her a safe place to go. And if you witness abuse, contact the police. Once they are involved, it may make it easier to get her out of the relationship.
I hope this has helped you,