You would be unreasonable to ask him to buy presents, when he has specifically told you that he and his wife have decided not to do that. They are generous to you in other ways. As is their prerogative. You don’t have to agree or like it, but you do have to respect their choices. You can adjust your gift giving/response in light of your views of this, but that is it.
The problem you have is that your children have linked getting gifts with being liked/loved, and lack of gifts as a sign of dislike, when this is not the case. They are children, but it’s not something every child believes and if they do there is a reason that message has been picked up. You say you don’t mind re the gifts, but I’m not so sure
that’s true. I think it will be hard to convince your children of something you don’t actually believe either, if you are honest.
I also think it’s a bad idea- if you do ask him and he does, what message are you sending your children? You’ve just confirmed that presents are a demonstration of regard or love, and a lack of one is the opposite. You are concreting in that fear/belief your daughter has, and frankly, that’s the last thing you should be doing given her sensitivity with regards to her step-father’s family’s gift-giving practices. It’s not helpful or healthy to her to take this track, even if it does (temporarily) cheer her up, thus making you feel better.
Your brother may genuinely just not be terribly interested in your children. It sounds like he and his wife are childless- assuming that’s by choice, it should perhaps not be a surprise to you that he is a bit indifferent about them. I’m sure he is generally fond of them, but may not really interested forming a closer bond. If they’re childlessness is not by choice, the situation is more nuanced, but still understandable that he might chose to have a more distant relationship with your children As long as he is not unkind to them, I don’t think there is much to criticise. He might not be uncle of the year, but he’s not causing harm. Equally, you can chose to decide how you respond to that indifference and may decide it has a negative impact on your relationship with your DB&SIL.
In his position, if you demanded/suggested a present to prove to a child I liked or loved them, I’d not be terribly impressed. I’d suggest you work out a way to demonstrate to your child the inaccuracy of that assumption rather than expect me to adjust my behaviour to appease the child’s misconception.