If DD and SiL have been "pandering to her" all her life, there's no point you trying to say anything against it now.
On various threads, on GN, people have said that something generally considered ungrammatical is actually part of normal speech in their area.
Don't allow your DGD's mood to dictate how you feel. It is disconcerting if she behaves illogically but it's best just to let it flow past you.
As others have said, your recent loss may be affecting your own moods and reactions. Please try to keep harmony with your family. 
How can I deal with this rude guy?
Good Morning Thursday 14th May 2026
When Is News Not Really 'News'?
When a political leader lies on their CV - can you trust them?
