Help me model this deal

[Rob Adams]

Subject property is distressed and gutted 2/1 in an up-n-coming historic area with small restored bungalows with high-end, tasteful finishes. Think college professor buyer. 

The ask is a reasonable $570K based on MLS comps, demo has been done down to the studs, footprint has been made for addition to 3/2 and 2 car garage.

ARV is a plausible $800k based on MLS comps. Rehab is $100k. Carrying costs $9k per quarter. Financing costs $3k per month. Closing costs $10k.

I'm putting up the money and obtaining the loan. Licensed GC is my brother and is running the rehab at wholesale and splitting the pre-tax profit on the flip 50/50. 

How do I make this deal work? If i go with a cheap seller's broker who does bare minimum I could get away with paying 1% when we sell...but it could drag on. Or we pay a good realtor 5% and she closes quickly, but takes 5% of an already extremely tight deal. And what if the rehab is slow and we have to carry for another quarter? 

Thoughts?