We're here for you Chey.
As for waiting till your MIL changes, that's never going to happen. She believes she's right. And she's spoiled because people give her what she wants due to her behavior when she doesn't get what she wants.
I was very close with a family that had a MiL that was a narcissist. It was all about her and she had six kids! Those children were abused horribly growing up because bratty behavior would reflect on her and her husband and cause people, all this in her mind, to think they were not good Catholics and not fit parents.
Consequently, they raised children who grew into adults that are terrified of upsetting her. Her husband died a few years ago but it was always all about her. This was a woman who would get upset with one of her kids, or her own sibling, and deem it fit punishment to not speak to them, look at them if they were in her vicinity, or acknowledge they even existed and for years! While she maintained her "right" to be mad at them and until, if ever, she decided she was over it.
Your MiL sounds similar to her. She looks at your daughter as if it is a personal affront to her side of the family that the little darling has issues with her eyes. It's all about her.
As it is about your personal schedule as a family when you're married to "her" son! It's her! All about her!
The most sacred responsibility a person or persons have is when they undertake the choice to bear children and raise them. Those little one's are the future and their upbringing is paramount. And as any parent knows, they're little sponges. They pick up on everything in their environment where they're raised.
You owe the life you want to give your children and the peace you deserve in the process to yourselves and your children first.
Your MiL has demonstrated for years that she isn't the one that believes she needs to respect that or you as a family unit. And what she fails to realize is it is a privilege to be invited into the family her son has created. Not a right.
If I were you this is what I'd do.
I'd ask her if she would have time to have a sit down and talk. Just the two of us.
And then I'd put it all out there. I'd also bring her pamphlets, even copies off the net, as pertain to Nystagmus and Optic nerve hypoplasia, so that she could elect to educate herself rather than stare at my child for some sign that will tell her what's going on.
And then I'd set boundaries again and I'd be very firm about that. (Of course review all this with hubby first). I'd stay strong, and I'd look her in the eyes the whole time. Even if I started to cry I'd look her in the eyes so that she would know I mean what I say even through the tears and the nerves that are there in part because of her. And if those tears showed up, I'd tell her why they and the nerves were there; because of her.
I'd tell her that if she hadn't done an excellent job in raising her son that I wouldn't have had an excellent choice in a husband and father of my children and her grandchildren. And I'd tell her that now that we are a family she has to trust that she raised him right and to know his mind and what he wanted in a wife.And that she was just going to have to trust it all to work out as it will.
And then I'd leave her with this: I hope this helps you understand my position and ours as a family. We are raising our children to the best of our ability and I trust that you know that any difficulties you present to that mission will only hurt your son and his children and the wife he loves enough to bring your family legacy into the world with.
And then I'd depart. And I'd hold my ground. But I'm like that. I hate bullies and especially those who bully children and women. Even when they're also women.
No one can make you feel insecure without your permission.
This is your family! Your responsibility.
Women like your MiL will not install boundaries into their behavior. They'll push as far as allowed. If you're intimidated she'll know that and she'll push more. How she'd raise your family means nothing. It is your family! You just happen to be married to her son.
The only thing she has responsibility for now is in how she accomplished that and trusting she did a good job as he raises his family in union with his wife using all the tools she instilled.
Stand your ground. Children should know their grandparents. But only when their grandparent(s) are a positive influence in young impressionable minds. Those grandparents can be either a blessing or a dire warning. And those who are a dire warning cause way too much drama and emotional pain that has to be overcome later because of the exposure they were allowed to those impressionable children early on.
Remember, her access to your children are a privilege the two of you grant her. It is not her right.
God be with you. Pray and ask for his guidance. He'll not let you down. And pray for your MiL too. Toxic family starts generations before. She's simply showing you how she was raised. And it is that influence that you don't want to affect your children.
She's older. She'll be out of your lives as God wills one day as all parents pass on. Don't make it a family feud, but also don't let yourself be bullied out of your faith and trust that you are qualified to parent those precious gifts God himself trusted you with.