My mother died and her will was incorrect. She had 2 children (one being myself) and had remarried, he had 2 children. They lived in her home, which was her separate property. The attorney that wrote her will put in the will that the home was joint property, and a third of the estate was to go to them. Can the will be nullified? Does a null will mean that just that part of the will is null, or does it nullify the entire document? By the way, her husband is also deceased, shortly after she died.

Wills are the wishes of the person having them written. To be incorrect they would have to describe assets wrong or you would have to prove that the wishes were different, very difficult.
It sounds like your mother left a third to the 2 of you, a third to her last husband, and a third to his children?
Some wills have a clause for different provisions if the spouse dies within X days.
If not, or it exceeds that date, then he got one third and that third is part of what he left in his will, most likely to his children, who now have two thirds.
It doesn't seem fair but it happens all the time, and although you can pay a lawyer to contest it, you will probably lose.

No, she left a third to her husband and a third each to me and my sister. The third is to pass to his two children. But the house was her separate property, but the will states that it was jointly owned, which in joint ownership, a half would have passed to him, which would mean that they also inherit another third, the intended third.

Louisiana is one of the few community property states, and houses are part of it.
Try http://www.louisianawills.net
There are some obscure terms in law there, and you can contest, but I am sure you will lose.

Louisiana is one of the few community property states, and houses are part of it.
Try Louisiana Wills (http://www.louisianawills.net)
There are some obscure terms in law there, and you can contest, but I am sure you will lose.

Im not sure why this was referenced. But it clarifies the OP's point of the home being separate property. Not community property. How was the deed titled as that would overide the will as a statement of fact. How was it addressed at the time of your mothers passing? Who handled the estate ?

The house was in her name only. She left him a third of it, but the will says it was joint property, which it wasn't. She had cancer and was on pain medicine and was, frankly, out of her right mind, which is another issue. I don't object to the third, but isn't the will a null will if the attorney drew it up wrong?

The Beavers did the will

It was purchased prior to the marriage and was never put in his name

So what you have is a conflict between what the will says about ownership and what how the deed is written. In my opinion, the deed will take precedence.

The will needs to be submitted for probate. And the deed needs to be presented as evidence. The probate judge will then decide whether to invalidate the entire will or to deal with the reality of the title.

You may challenge the will in probate court, the entire will may be challenged, or part, you will need an attoreny for this, so hire one quickly to file inprobate