Legal Question in Criminal Law in California

This is a really, really, really bad idea.

No, you cannot do jail time for your brother. If the jail staff realizes ]you showed up and claimed to be your brother, you will BOTH be in even more trouble: you for impersonating your brother, and your brother for not showing up to do his time. Then you'll probably wind up in jail together.

What's more, if there are people in the jail who are out to get your brother, they'll probably be just as happy to take it out on you.

If your brother is concerned for his safety, he should let the jail staff know so they can house him appropriately for his protection.

No, we do not have the "designated hitter rule" in our penal system.

Of course not. The reason your brother was sentenced was to punish him. Letting someone else could serve the sentence for him would defeat that purpose.

Only if you plead guilty to the crime and 'exonerate' him somehow.