No-you can not change a will after death.
If you have clear proof-this would be improper
and could be corrected in the probate court and possibly a
The answer given does not imply that an attorney-client relationship has been established and your best course of action is to have legal representation in this matter.
What you seem to be saying is that your father (now deceased) left a piano to your brother with the condition that the father's surviving wife can use the piano during her lifetime. The surviving wife does not want to keep the piano (since she has no place for it) and has given the piano to the brother.
If that is what happened, I do not see anything that would be improper. Your father's will likely does not require that his surviving wife keep the piano until she dies but provides that she can have possession of the piano while she is living if she wants it.
As for "your" painting, she does not have to give you the painting if the will provides that she can have it while she is living. Just because she releases her right to the piano does not mean she must do the same to the painting unless the will provides for that.
It is not clear from your post whether the probate proceeding for your father's estate has finished. If the probate has not finished, the surviving wife may find a way to assert a claim to the painting. If the painting is community property, your father cannot give his wife's share of the community property away in his will.
You should review the specific facts with your attorney to see what legal options you may have.
If I understand your question, your father's will indicated that certain items would go to you and your brother upon the death of your father's wife. Then your father's wife decided to give the item that was to go to your brother, the piano, before her death, but did not do the same for you. Correct me if I'm reading that wrong.
I do not think you would have any case for getting your inheritance early, since she is still abiding by the terms of the will as to your inheritance.
In the case of your brother, your father's wife is essentially giving your brother his inheritance early. Whether that is legal depends on the terms of the will. If she was to have use and control of the piano until her death, then it would seem that it is up to her whether she would like to move the piano to your brother's residence to suit her needs. That is not necessarily changing the language of the will.
I am licensed to practice solely in Illinois, and do not intend to give legal advice or create a lawyer/client relationship. I highly recommend consulting an attorney licensed in your jurisdiction to protect your interests.