The thing is- by reacting in this way, you make yourself sound unreasonable, where as your brother will be the fun & cool uncle. This is a dangerous precedent to start & one you will regret if you don't learn an alternative way of handling these types of situations before they are teenagers.
There are always going to be influences in your child's life, you can't protect them from all of them. You can't put them in a bubble, especially if you want the option of leaving them with other people sometimes or eventually letting them leave the house on their own, trusting them to make safe choices.
Instead of focusing on the actions of others, teaching your child about making good decisions. By calmly & rationally thinking & discussing about what they enjoyed about their experience with their uncle. Talk about why gambling can seem fun & exciting initially but become an awful & dangerous thing for some people & why that is. Teach them to consider what people are telling & showing them & think critically about whether it matches their values.
By making such a harsh judgement on your brother & not acknowledging the positives of what happened you stand to damage your relationship with him & potentially your daughters. Your brother actually shared a special & lovely bonding day with your kids, although not in a way you approve of, he maybe wasn't aware of this before doing it.
They clearly had a lot of fun, so you shutting this down instead of using it as a learning opportunity will only drive a wedge. Making you look unreasonable & certainly taking away likelihood that they will trust you about the negative aspects of gambling...