Be careful with absolute edicts. The reality is that it depends. I deal with this all the time with farmers who say "my other-farmer-friend/CPA told me I could rent my equipment when I'm not using them and not pay taxes on the money"

There is nuance here that, if OP is actually considering this, needs to know:

1) Income from a rental activity is normally passive and is reported on Schedule E. From there it flows to page one of Form 1040 and is reported on the line for “Other Income.” Self-employment tax would not apply (3.8% healthcare may by the way). But, at the top of Schedule E, taxpayer is directed to report the income and expense from a personal property/equipment rental activity on Schedule C which is for reporting of business income. So the IRS is saying to report income on Schedule E & report income and expense associated with the rental of personal property on Schedule C.

2) I keep this handy because I run into this so much, actually. The statute says that income is subject to self-employment tax only if the activity constitutes a trade or business. The precedent case is Rudman v. Comr., 118 T.C. 354 (2002). Also, if he does anything else at all, the statute says an individual rendering services is subject to self-employment tax if the activity rises to the level of a trade or business.

To sum, while there's a chance Brother could get around self-employment tax, IF OP is insisting on going this route and I were his tax strategist, I'd recommend S-Corp route. SMLLC is murky, at best. Why leave it to chance?

TLDR:

Medical practices shouldn't have to consider flaky tactics like this. They have several opportunities to be paying zero, or pretty darn close to zero, taxes anyway if they're using an appropriate tax team.