i let my brother sign into my icloud account on his old phone until he gets a new one , is he going to be able to see messages and pictures and stuff??

If his iPhone is using your Apple ID then various services just see it as the same one person using the phone. So yes, he can see your photos and Apple messages and whatever else uses your Apple ID.

There is no reason why he should not have his own Apple ID. An ID is not attached to a particular device, it is associated with a single person. You sign in on any device you own and that, in essence, makes that device "yours".

Create an Apple ID - https://support.apple.com/HT204316

https://appleid.apple.com/faq/#!&page=faq

"How many Apple IDs do I need?

Just one. Use the same Apple ID everywhere you sign in to ensure that all your Apple services and devices work together seamlessly and you can access your personal content from all your devices."

Anything that your brother sees will be what is in your iCloud account if you let him sign in to your account as your statement says. Why would you let him sign into your account?

I’m still confused as to why he would want to sign into your Apple ID. If he is signed into your ID, he will see your content, not his own. Can you clarify a little more?

Your brother should get his own Apple ID. He can use that ID on a new phone when he gets it.