I'm sorry to hear about your brother.
I'm sure your brother had his own iTunes account. All the apps that came with the iPad will be associated with it. There is no way for you to continue using and updating those apps with your own account.
Instead what you need to do is restore the iPad in iTunes so that you can use it with your account. This will mean you lose all the apps currently on it. Any apps you bought through your own account you can reinstall at no cost. When you go to the App Store it may look like you are buying them again, but after you hit buy you will be told you already own it, and be asked if you want to re-install it.
As compensation you will be able to instal many of your iPhone apps on the iPad and use them. Most will probably only work in an emulated mode, either half size in the middle of the screen or at 2X, but with poor resolution. Some however may be universal apps, and will work fine on the iPad, possible with a few extra features.