Hi Lost, your mother might be struggling, but that's not down to you. It seems hard to say, but if your brother not baptising his child sends her further downhill, there's a lot more going on than external events. Been through the baptising scenario in my family BTW, everyone recovered fine and I'm sure my Mum would now look back and wonder what she was thinking.
As for 'disappointing' her, think about what you mean by that. The word disappoint implies you've let her down, but this is really about your own family and what's best for them.