It sounds like your mother's sister bullies her and your mother tries to remain passive as she most likely learned during her life that her sister just bullies more if she doesn't get her way. And what you experience with this Aunt is something your mother has dealt with her whole life and probably learned it's useless to try to stand up to her sister and her sister is stingy about apologizing for behaving badly.

Now that you are older you are seeing the unhealthy dynamics more and how your mother tries to be more passive as she dislikes confrontation. It's probably something she learned growing up around her own family dynamics and knowing her sister gets bossy and controlling, something you are becoming much more aware of.

Perhaps there is a part of you that already knows your mother prefers to avoid in an effort to reduce feeling stressed. This is part of her desire to not make her falling a big deal too. Yet, as you found out your brother gets angry and insists you let him know when something like that happens. What you did was honor your mother's wishes to keep it quiet, yet even if you mention that to your brother will respond with don't listen to her. That puts you in a bad place which can lead to your feeling like the punching bad simply because you felt it was your mother's right to tell whomever she wanted to about what she experienced. However, her motivation is more about getting to spend time with her grandchild and she doesn't want anything to interfere with that. And perhaps your brother and his wife are having problems and your mother is trying to stay neutral so she doesn't threaten being able to see her grandchild. My guess is this is how your mother learned to navigate around difficult people and part of why she tends to bring up history to prove how others only get more aggressive when any effort is made to stand up to them.

If your birthday is coming up, it's YOUR day and you don't have to make it into some kind of family thing.