Guy is booked into the jail. Signs his brother's name and not his own on jail paperwork. Clearly a misdemeanor A. But is it a felony? Did he intend to defraud or harm another? Can the government (the jail) be "another"?

I think the government was possibly a defrauded party, and I believe the government can be "another".

Also, you could make the argument the brother was harmed. He was clearly not subject to the arrest or criminal penalty, but he could have been subjected to some or all of the disabilities associated with it if his brother were successful.

From what you've described, I think you'd have a hard time establishing that there was intent to harm or defraud the State, just from the definitions of harm and defraud. You could try making the argument that by giving the false name, he intended to place the State at a disadvantage when it came to investigating or prosecuting him, which could fall under the definition of harm. However, that seems like a really tenuous argument to make to a jury.

It sounds like it would be an even harder case to make regarding the brother being the "another." Unless he intended for brother to wind up being put through the prosecution, etc. (as opposed to using brother's name to throw law enforcement off his track), I don't think the mere act of using brother's name means he intended to harm brother. If the mental state allowed for knowing it would harm or defraud, or for being reckless about whether it would harm or defraud, the enhancement would be there, but I don't think you can get past the intent requirement in this scenario.

what definition are you referring to
I see the presumption in 32.51 but not the definition
I think this clearly falls under 32.51

32.51 looks more appropriate than 37.10 in this situation