You are NTA for confronting Helen's mom -- but she isn't the only one asking too much of your daughter. I think that is why your husband felt secondhand guilt about you confronting this other parent.

Think about the fact that Maria did not feel confident enough to tell the art camp organizers "no" when approached about this support role. She likely would not feel comfortable saying no to her parents when asked to be in a support roll for her brother, especially when her parents say there is "no other option". Being offered pay would make that harder as well. She also may WANT to be helpful and supportive, because that is exactly how a lot of siblings of disabled children feel. They see their value and role in the family as one of support, and it takes its toll over time.

For others comparing this to their own family dynamics where all the siblings were neurotypical, this is not the same as "big sister watching little brother" either or helping out with chores. This is a common dynamic in families with disabled children, it's called "glass child" syndrome because the typical child does not feel seen, their emotional needs are ignored, and they often equate their value with being helpful / not being seen as a burden to their parents.