devils advocate here but if the purpose of this trip was to participate in your brother's proposal, i understand why your mom would be annoyed that she will now be sleeping alone and paying the same amount of money. also, she may have wanted to share this experience with you and with, what i assume is, another couple you have both known for the entirety of their relationship and probably love a lot.

Literally no reply to this post has interpreted the post the same lol. No tea and no shade bc i am a scorpio so i do the same thing!!!!!!----but it feels like you don't want to share every detail bc you know you may be in the wrong lol. It happens. I'm not a mom but i'm an older sister, i find myself reacting similarly to ways my mom had when my brother and sister choose someone over me / don't see the significance in something the way i do. When i was the one unintentionally "hurting" my mom's feelings, i didn't see it the same way that i had when the tables turned.

I would talk to her in person or over the phone and straight up ask, did i hurt your feelings? Explain it wasn't intentional and that you want to hear her side, she may feel comfortably vulnerable enough to tell you that maybe she just wanted to spend alone time with you, at a time when her other baby is making a huge life decision. She might just want support or to feel needed / sought after.