Your MIL can only make decisions for you if you let her. You can also listen to her words but you don't have to hear them.
There is shed loads of stuff on Google about the tension, jealousy, tug of war etc. between two women for dominance over son and DH. Why not read some of it? It is a very difficult relationship to get right.
Before you get too worked up about her ask yourself some questions?
Do I really care what this woman thinks or feels? If the answer is ' no' then the next question is why can't you tell her in an assertive way to back off?
Life is short and if you don't find some confidence from somewhere to tell her how you feel, then you will get angrier and angrier.
It is amazing how we can forgive a friend for saying or doing something annoying but when it comes to our MIL's we are struck dumb.
Your MIL is someone who loves your DH. He has been there all her life. You have been thrust upon her. She may feel all the same emotions as you including grief for the loss of her son, to you. Because believe me more you may not realise it, but you hold all the cards.
The only way to have a relationship with her is to be assertive and TELL her. Being assertive gives you and her the opportunity to leave a conflict with dignity.
Unless she is demonstratively mentally ill you need to approach her yourself. Moaning about her to your DH could destroy your marriage as you may be complaining to him about someone he loves. (Your post was brief so we don't know about his relationship with her)
Would you be pleased if he slagged your DM off to you and said he hated her?
If you haven't got the confidence to speak to her then get counselling and do a mindfulness course. If you can't afford counselling then do a counselling course at a local college, ( it is a much cheaper option) it will do wonders for your self esteem.
Best of luck xx