Yes, but there are very important side effects you might want to know.
[Next three paragraphs are obsolete because further case detailing, but are relevant as the original case is still readable].
He has to either sell shares to you1 or give away shares to you2.
There might be, however, family business law applicable, making an exemption from taxes quoted in the footnotes detailing the previous paragraph. Ask your father whether he knows anyone or not.
And from then on you might be responsible for taxes imposed on the company (go ahead with the TS;DR). That’s a side consequence.
If the workshop or office is free from VAT, I don't know why would anyone have to pay taxes from the place's acquisition. You would normally pay ITP-AJD from that transaction, but it certainly [(article 6)] looks like nonresident nationals are free from it.
The important bit is that, as wrote in the previous answer's version, you can be called co-responsible for company's taxes if things go wrong.
1: Then you pay ITP-AJD taxes for that, because of its article 6.1 A) if you came back to Spain (or you appear in census fiscally in Spain). I don't say you're not subject to some other tax or some other case of this tax. Not to mention maybe you are not subject to this, but your brother might be.
2: Then you are subject to ISD taxes because of its article 3.1 b), read article 20.2 c) too as a confirmation that LLC shares are subject to this tax.
He told us that first we would need to be registered as self-employed workers, then this business would be put in our name and every month each of us would have to send to a bank account (local to my home country) 95% of the revenue. My brother and I would each keep the remaining 5%.
[Next two paragraphs are obsolete because further case detailing, but are relevant as the original case is readable].
All of this is known as simulation, at least in the Spanish legal framework. That's something you might want to search about.
Ask your father how is he going to sell [which percent shares?] of his company to you and your brother, and for which price (of course don't tell here).
[...] every month each of us would have to send to a bank account (local to my home country) 95% of the revenue. My brother and I would each keep the remaining 5%.
My father's new business is not yet active nor registered (as per my knowledge). His plan is to make my brother and I owners of the space where this business would take place, and then we would register and start this business without legally involving my father.
You would be owner of a company, and its office or workshop, but not of its revenue. That could be argued as a case of absolute(resource in Spanish language) simulation. You want to read article 43.1 g) and h) of Law 58/2003.
Your Spanish income might be subject to IRNR, maybe not. If a very large income, then most likely. We're discussing percentages, so nobody can know for sure. But that's better, for too large quantities could even make enough bulk for a criminal offence (in case not declared). So if it feels like a lot of money (what you earn in clear numbers, not what you de facto keep), then you might want to make sure you tax pay it3 (probably making sure your father knows altogether this plan of yours).
You could eventually [my guess] even be charged with ISD for that revenue transfer back to Spain (if the receiver misses to pay them), if you're not co-owner of the bank account.
3: So you might end up with net loss! Depending on tax rates [am not familiar with].
I am basically clueless about the laws in this regard, while he is a lawyer. [...]
Then make him teach you whatever necessary until you trust him more than Stack Exchange.
Is this a legal way to avoid extra taxes or is this a way for seeking legal trouble?
Possibly both. I think you shouldn't approve este negocio if you don't know (1) what's the current accounting situation of that company, (2) the current taxes situation of that company, and specially (3) what's your participation (and your brother's, and your father's) quota at any point in the company's life, from which some of the legal responsibilities might be derived.
In general, you want to read section 3.a "responsibles" of chapter II "taxpayers" of title II "taxes" of Law 58/2003. That's only for the truly worst of cases. In a wonderful world it's an overcautious baseline. But if things go wrong; and this could not necessarily be caused by your father, but by partners/associates/co-managers/co-de-facto-owners of him, or even mid-tier managers or high profile employees; it would be the appropriate base line (only tax related, not considering other kind of civil responsibilities that could apply).
If the company does its taxes alright, there should be however nothing to worry about. For me all of this can be narrowed down to two-three essential things: (1) trust in your father (and his future employees and his future potential partners/associates/co-managers/co-de-facto-owners), (2) how much are you wanting to play things safe or risky on your side, and (depending on earnings volume) (3) how likely are you going to be subject of IRNR investigations, or your money transfers out of your account be investigated because missed ISD duty (if whoever receives misses a large ISD duty).
DISCLAIMER: This is not your legal advisory, it's a vague, yet somewhat informed, entry level reference in order to guide you. Here's no responsibility for any damages caused. If on doubt, always choose to go hire a lawyer.