If 'they were still legally married, then by law, she did nothing wrong, legally speaking.
If your grandfather did not have a will and was not married, then his children should have gotten his estate. If any of his children died before he did, then his grandchildren should have gotten their deceased parent's fair share of their father's estate.
If you are a minor, there is nothing you can do. Ask your parent if there is something s/he can do.
Even if your Grandmother is evil, as you say, it is not illegal to be an assaholic and if she was still legally married to him when he died, then she gets all of his estate. I am sure your parent knows that because nothing was done before his house was sold.