To be honest, it sounds like you are probably the worst person in the world to 'have a word' with her - you sound as though you don't trust her not to hurt her own baby, and you are already imagining her being overwhelmed when this baby, who isn't even born yet, is a toddler. You are envisaging a worst case scenario, whereas the truth is that most parents learn to be patient when their child is a helpless infant, and that's how they get through the toddler years and beyond. Your concerns are completely legitimate, and I'm sure you mean well, but banging on about how you 'want her to be the best mum possible' sounds incredibly judgemental and as though you are in some way responsible for her, which you aren't.
A 'reality check' is probably going to sound patronising (and sorry to say it, your posts do come across as quite condescending about her abilities and how much of a role you should have in how she parents), so perhaps framing it in a different way would be a good idea.
Could you ask if there is anything you can do to ensure that she is getting enough time to relax, come up with a plan of things you can do for them both (because it seems like you are completely absolving your brother of responsibility for any parenting) to make things easier once the baby is born e.g. batch cooking for their freezer, going over one night a week and just holding the baby to give them a chance to eat a meal together, organising a rota of family to visit and help out in the first few weeks or similar?
I think if you approach this from anything close to the perspective you have outlined in your posts, you will risk causing her more stress and alienating her and your brother. Be on her side, ask her if she's planning to go to post natal groups or similar and find out information about less formal support networks so that she has people to call on if she needs them, and accept that she might prefer not to ask you for help.
Good luck - you sound very caring, so if you can ensure that is the thing that comes across rather than your concerns, there's not much more you can do.