I'm with maddyone on this, having been in the same situation ourselves, being invited to a family event, along with my father. My father lived alone, with only a little help from a daily care team, as he was fiercely independent. However, just because he lived independently, in his own home, didn't mean that it was easy for him to attend a family event. The logistics of it all were a nightmare ... for me! He sometimes lost his balance, and had nasty falls. He walked with a crutch, and struggled to get up/down stairs (we had a stairlift installed for him). He didn't handle long car rides well. He tired easily, and sometimes felt dizzy and would need to lie down. Whenever we took him out/to a restaurant, I was constantly on edge, not knowing if the chair would be right for him to be comfortable, and I would need to follow him to the toilet for fear that he would lose his balance ... although he always insisted that I didn't need to, but we've had too many trips back and forth to hospital to know otherwise! If there were any stairs to navigate, it was a nightmare. He always insisted he could manage, but it was me who always had the stress, worrying about him, and was never able to enjoy the occasion.
Our family event was a 2 hour car journey to a countryside location. We had to stay overnight, but the only hotel available close to the venue didn't have a lift, and the bathrooms didn't have a walk-in shower (impossible for my dad to get into a bath!). At the venue (a large converted barn), the welcome drinks were held in an upstairs lounge/bar area, with very little seating, and the toilets were all upstairs too (no lift). The dinner/evening entertainment was held downstairs, in the main barn. We had transport arranged to/from the hotel/venue. Had my father needed to leave early, getting a taxi to the remote location would've been difficult.
The OP states that her mother is frail, and she tires easily. She also states that she, and her siblings, provide most of her care, so one would assume that she needs help of some sort. With a long car journey, and an overnight stay in an unfamiliar hotel, I would say that the logistics of everything need to be taken into consideration. It is not just a simple matter of whether or not her mother would like to go to the event, or whether certain family members think she should go. The onus lies with whoever will have to make all of the travel/accommodation arrangements, and take care for her mother during the event, and that person will possibly not be able to enjoy the occasion themselves, due the stress it involves.
I never suggested to my father that he shouldn't attend our family event. On the contrary, I reserved a hotel room for him, and made all of the arrangements to take him with us. However, at some point before the event, he did decide for himself that realistically it would be too much for him, and declined the invite at the last minute. Having attended the event, I can say that it would've been a logistical nightmare for him ... and me!
Portmeanne, I would advise you to take everything into consideration. How much help does your mother require? Is she able to stand, if required. Is there somewhere she can sit/rest comfortably if she's feeling tired? Is the event going to be noisy (wedding?), and she prefers peace & quiet? Would she need assistance getting to the toilets ... where are they located? Can she easily get back to the hotel alone if need be, or will someone have to go with her (you?), and miss the rest of the event?
MissAdventure, I don't think it's a matter of it being outrageous and reducing an adult to being treated like a child. It's simply about being realistic. Unless people are in the same situation, or have experienced the same, it's very easy to assume that others are being selfish, and only thinking about themselves, and not their elderly relatives. I loved my father dearly, and would've done anything for him, but I also knew his capabilities, sometimes better than himself ... or rather, he wouldn't admit when something was too much for him. Of course, if the OPs mother can attend the event, and it doesn't cause any stress/problems to others, then by all means she should attend.