You can have it while a sole-proprietor, corporation, etc.3.

For your flip property scenario, flip profit is taxed as ordinary income for both the pass through LLC and for the sole proprietor.  

For a sole proprietor business with net income around $50-60K, if I like to include one family member(brother) in my solo 401K, what will be best structure to be cost effective and tax efficient?

What about just staying a sole proprietor?

(This can be done as a sole proprietor as well)Fifth, Investing in a flip inside and IRA can be a trouble as you can run into UBIT issues AND you can run into a problem of the TP needed actual cash and money personally not in retirement.NOW, my BIG issue with your statement and you as an accountant even posting on a real estate investment board should know this as Tax 101.

That being said there are many sole proprietors that are ex Big-4 and may serve you well for less cost.

@Brandon HallOn page 1, 2nd column of the IRS Instructions for Form 1098, under Exceptions:You need not file Form 1098 for interest received from a corporation, partnership, trust, estate, association, or company (other than a sole proprietor) even if an individual is a co-borrower and all the trustees, beneficiaries, partners, members, or shareholders of the payer of record are individuals.One of my lenders (where the loan was secured by a property) sent me this from her CPA, saying that she's not going to send us a 1098 and that we have to send her a 1099-INT because our company is an LLC.My CPA said that we're just going to send her a 1099 to be safe.  

I currently own 4 duplexes and 1 sfh as a sole proprietor.

Schedule C if you a sole proprietor) & you do not have any w-2 employees working for your self-employed business or otherwise.2.

I am a sole-proprietor but I still have a day job.