T.R.
J., my advice has nothing to do with whether your brother is bringing a Pit Bull or a Chihuahua or a (insert breed here). It has everything to do with the safety of your children, & the fact (as you presented the story) that your brother's dog does not have experience around children.
Dogs who have not been raised around kids cannot be trusted with them. Children smell funny, move unpredictably & make loud, sudden (often high-pitched) noises, all of which makes unaccustomed dogs very uncomfortable.
Too many people do not "read" their dog properly to know the signs and signals that are being given that a dog is not comfortable in the situation, until it is too late. I cringe watching the videos people post of their dogs fighting with themselves to -not- lash out & bite children, while the owners comment on how "good" the dog is with their kids.
In this case, your brother's dog will be out of it's element, in a crowded house with lots of activity, and 2 beings it's never met before, much less been trained to behave around.
I don't know the reasons for your brother bringing the dog. Is he traveling, & staying with your sister? Just can't bear to be away from his pooch for 4 hours? Doesn't matter. You will need to be very careful with your children, and if there is any sign that the dog is stressed, then it needs to be secured in a different location, whether that is in a crate, or shut in a bedroom.
Signs of stress - panting, whale-eye (showing the whites of the eye), refusing to look @ the "object" (children), retreating, whining, tucked tail, "tight-lipped" mouth, raising fur, growling, showing teeth, etc.
Let me be clear about one thing - a dog showing stress signals around children does NOT mean that the dog has a poor temperament. It is NOT a reflection on whether or not it is a "good" or "bad" dog. It means that the dog is not familiar with children, does not know how to respond, & is going to protect itself first. Some dogs do this by retreating, others by asserting themselves. Dogs that want to retreat & cannot, will assert themselves, out of protection of their space.
So, I urge you to talk to your brother & express to him that the safety of your children and the comfort of his dog must come first. At 7 months of age, if they are in carriers/pack n play/etc. they should be fine - I would refrain from letting the dog around them unsupervised. If they are crawling around, the dog should be removed.
Depending on how often the dog might be around your children, you might want to allow for introductions. But honestly, if it is 1-2x a year, it's not worth it.
One very important message - do NOT reprimand the dog for exhibiting stress behaviors. The last thing you want to do is have the dog think that they are not allowed to growl, etc. & have them suppress this response & lash out with "no warning".
For more information on how to safely handle dog/children interactions, check out this article. http://doggonesafe.blogspot.com/2012/01/my-dog-growled-at...
You might even want to show your brother this post & open a dialogue about how to best handle the situation. Good luck! T. :)