Your cousin's brother is being exploited. Let's just say it clearly. He's making enough to send $400 home, sure, but in California that means he's probably making minimum wage, working overtime, living with five other people, probably in a car sometimes, getting no benefits. His labor is worth more than what he's getting paid, and the only reason an employer can pay him that little is because he's undocumented and desperate.
I don't say that to be harsh about him - the system is harsh about him. But everyone in your post kind of glosses over this. Your cousin's family is better off. Cool. But what about him? What's he eating? How many hours is he working? Can he ever get healthcare, or does he just live in fear that a minor accident becomes catastrophic? Does he ever get to actually build a life for himself, or is it just about sending as much as possible back?
This is why both sides of the debate actually do need to reckon with remittances, but not the way you're describing. The restrictionists should realize that their preferred policies would just make people more desperate and more exploitable. The open-border folks need to admit that legal status actually matters for worker power - you can't have a functional labor market where one whole class of workers can't report wage theft, can't complain about conditions.
So yeah, acknowledge the remittance economy. But do it in a way that asks: how do we make migration a choice instead of a desperation play?