I assume you are talking about business insurance so should you get sued for something you would have coverage..... or should you have some issue (fire, flood, etc...) your inventory/machinery costs would be reimbursed....
That said, if you and your brother are both young, have no appreciable assets, and haven't even made a sale.... I'd skip it.... honestly, unless you hire some employees, or start doing something where you open yourself up to the public entering your "store" (not a problem with eCommerce)... I wouldn't worry about it.... (this assumes you are like a typical start-up and have less than 1K or so in inventory and only a few small machines to get rolling... if you have a large inventory and several thousand tied up in presses,
DTG printers, computers, etc... you have to consider the cost of replacing the items v. the cost of a monthly premium... on an aside, if you operate out of your home... or your dad's... your homeowners may cover some of the items if they aren't specifically used solely in business... )
Back to the point of the potential lawsuit.... off the top of my head, I would assume the only lawsuits you might encounter would be for infringement or trademark violation, and usually you get a Cease and Desist letter first.... other than that, I doubt your product is going to hurt or injure anyone... and if it did, the plaintiffs would likely sue Fruit of the Loom or whoever made the garment as their pockets are much deeper....
All in all... if you have no assets... there is nothing to take... or to sue you for.... and if you have some insane liability leveled against you from some bizarre suit.... bankruptcy is always an option.... without assets, and without a job that pays a huge amount of $$.... the court will generally let you file Chapter 7 and excuse all debts.... this isn't the case if you have substantial assets or a giant income, then Chapter 13 is more likely.... but honestly... this probably isn't likely going to be a concern....
I'd open the shop.... see how things go... and if you get to generating several thousand a month or so... then pick up some low cost insurance.... I ran my t-shirt business online for 5 years before I got any insurance whatsoever.... and now I only have it because I moved from printing in my garage, to a small warehouse where I have several thousand in inventory....
Good luck...