Given this context I simply wouldn't bother with her.
She stopped talking to you because you did what was best for you and your grandson when you had both suffered such a massive loss?
After the funeral of my daughter I had a 'friend', who had been to my sons funeral a few years previously, get upset because I didn't invite her to my daughters funeral (she never met my daughter). She called me selfish etc, and said I should have had the same people at both of their funerals.
She was my oldest friend, and I simply walked away from her. Her attitude was so shitty, and I had been through so much, I didn't need any additional crap from anyone.
It wasn't your finest hour commenting on that photo, of course, but she had made the worst time in your life, even more crap, and withdrew much needed support because she felt snubbed, so I absolutely understand why you snapped.
Even at this point in your grief you need to be surrounded by people who will be supportive and understanding and not centre themselves, your SIL is not one of those people 💐