About the cheapest option at this point is for the sister to go to a consultation with an estate attorney that also handles litigation.Make sure the sister brings any and all documentation she has.

@Sergey Tkachev@Wayne Brooks said it best - I would consult with an association attorney, especially one that knows co-ops and see what options you have. 

I really think you should consult a lawyer with these questions.  

As a solar energy consultant for SolarCity I have been working with portfolio investors, helping them add value to their residential rental properties with clean solar energy, without tying up any cash flow.

I knew I wanted to be a landlord some day so I started as a leasing consultant for a multi-family REIT renting apartments in Uptown.  

You should consult with a CPA vut I believe you have 180 days.

Market to investors, cash buyers for the last 5 years, consult a couple of realtors, maybe you can find some folks in your area on BP! 

Charge you for a long contract of sale with contingencies in itConfuse the dealYou may want to consult with someone smarter than a lawyer - like an experienced investorIf lawyers were so smart, they'd be investors!

To address the type of flooring material, first off, if you want a "luxury impression," stay away from the laminate fake wood.I would consult a local expert, but that said, I understand engineered products--with a substantial veneer thickness that's able to be sanded and refinished--are both functional and comparatively inexpensive to install over a grade-level slab.In terms of color, this is always subjective (and depends on what's going on with the kitchen finishes), but I would recommend a mid-tone finish for a flip.

I also haven't consulted an attorney on this and am wondering if having investing (I buy houses cash type marketing) and licensed sales marketing on one website is too much of a conflict of interest even if disclosures are in place.