Hi and welcome to JA. I am Ray and will be the expert helping you today.
Unless there was a will here that was probated then you have an interest in the property and they need you to sign and you have right to your share.
I would start with checking to see if there was probate
filed in the county she deceased in.
Laws of intestacy
spouse and descendants from you and someone other than that spouse
spouse inherits 1/2 of your intestate property
descendants inherit everything else
So he has a half interest and half of her interest.You and any other siblings split a fourth.
Check here with register of wills in her county of residence to see if probate was filed here and get copy of will if there was one.
You may well be an heir
here , see what you find out.They would need you to sign here if you were.The POA ends at death no longer valid, the will or laws of intestacy( no will) would determine who gets the house or proceeds.
I appreciate the chance to help you today.Sorry for your loss of loved one.Please let me know if you have mroe follow up.Thanks again.