She explained her baby doesn't settle when she uses a cover while breastfeeding, and won't take milk when she uses a cover.
"Feel free to let me know if I'm in the wrong here," she posted online.
"My baby won't nurse with a cover, but I do my best to feed him as discreetly as I can. My brother's wedding is this weekend and we had this conversation following my mother informing me that I should plan on feeding Freddy in the restroom ...
"My brother has been an a**hole" to me our entire lives but I've been trying to build a bridge with him for the past 3.5 years. I guess I won't be attending the wedding."
While she always tries to feed discreetly, apparently her efforts aren't enough according to her brother and mother.
The sister then posted screenshots of her conversation with her brother, which escalated to the point of threatening to call police.
Sister: "If I'm not welcome to feed Freddy at your wedding I won't be attending."
Brother: "You're welcome to feed Freddy but you have to use a cover."
Sister: "Then I won't be attending. I hope you have a great day."
Sister: "BTW it's actually illegal to tell a nursing mother she has to use a cover."
Brother: "I don't give a single f*** about you."
Sister: "I'm aware. You've made the abundantly clear our entire lives and every time I think you've grown up you do some s*** like this."
Brother: "If you think I'm not the grown-up then that's ridiculous. You can't respect the wishes for my wedding. This isn't your day. It's a day for me and [my partner] and we want everyone to feel comfortable attending. So be a f***ing grown up and make it so everyone can feel comfortable attending."
Sister: "Freddy's health and comfort comes first to me, always. If he is not welcome to eat in the manner that he is most comfortable, I will simply stay home with him. You don't understand because you're not a parent."
Brother: "Goodbye. Never talk to me again. if you show up at my wedding, I will call the police."
Readers took to Facebook to give their two cents, with many praising the woman for her stance.
"Wow, your brother sounds like a jerk. Given that feeding will be such a minor part of the day, you'd think he wouldn't have an issue with it. Especially given it's a family event," one person said.
Another wrote: "Sounds like a win for both sides. She doesn't have to attend a boring wedding with a brother she loathes. He doesn't have the sister he dislikes and her little goblin."
However, some sided with the brother, saying the sister could discreetly take her baby out of sight before stepping back into the wedding.
"Even though your brother's request is relatively selfish, you'd think you could step out for five minutes in a bid to keep him happy and help continue to build the relationship."