First of all, congratulations on such a wonderful opportunity. Perhaps making a list of questions you would like answered about the business would put you at ease. See how you feel about the answers then move forward. Sometimes these things have to be on faith, though. Best of luck to you! It sounds as though this is something you really want to do for your family.
Keep us posted.
There are a lot of positives and minuses of owning your own business and rather than write a 10,000 word post, ask specifics and I'll expand on those questions (as Musiclover suggested).
Probably the best thing you posted was you would be "employees" for a while, that means you get to learn the business, and shortfalls, while earning a known income.
I was employed by others for a lot of years, was basically COO of a small privately owned chain of stores for 17 years, then ran my own business for 18 years (got rich then went broke) and now I'm employed by someone else....so goes the cycle 
I agree with what SteelerNYC posted, it's 7.65% X 2 for FICA (social security and Medicare), add in unemployment insurance if you plan on adding any employees and possibly employee heath insurance, rent, business taxes, personal withholding which you need in to send to the IRS/State in advance and of course inventory, health inspections (they cost if you don't keep the "food place" spotless), pest control (not talking about relatives). Benefits, a LOT of things you can write off like a car/truck for business use - put a sign on the door of the vehicle, cost of all those things I listed as expenses (not so much the payroll taxes other than business license, etc). You can write of up to 25% of your house if its used for business only, think storage of supplies, business office for computer, files, desk, etc.
If the business does well, you'll do well, if it doesn't you'll go broke. Now as far as "promises" a lot of small business start with two "partners" one says they'll put up the money, the other says they'll do all the work and it's all great dividing nothing but the idea, the issues come up later when the business is up and running, then salaries, investment capital, profit split, etc etc break up many partnerships. Expect to work 60-80-100 hours a week as owners, be willing to scrub floors and clean public trash and bathrooms. Expect your only part time employee to "get sick" on the busiest Saturday of the year (and you'll probably see him/her at the pool/beach/bar later).
Believe it or not you CAN do very well, but chances are you won't have time to enjoy/spend it - you will be your own boss which means the buck stops with YOU when your freezers that told all your inventory have a compressor go out on the hottest day of Summer.
Build a history with a local bank and you'll have no problem getting business and personal loans, auto loans, their credit card, etc. - It will take a year of two of solid store deposits for your bank to trust you. Oh, and if this crowd (forum members) shop there, expect to pay swipe fees on every $1 sale you make....they don't carry cash
Edit/Add: I can't comment on the family relationship, that's your call and frankly if it goes "bad" will you be able to deal with it short term/long term? Again I'll say the best thing offered in this "deal" is the chance to be "manager-employee" for a year or two, you'll learn the business, the good and bad, and you can walk away without mortgaging your home and soul for a business you may or may not like.
Edit/Edit: I really should not post after working a 14 hour day - seems spelling and typos become the new normal 
Just to give you one extra thought: My personal business I ran out of my home from 1989 until 2008 or so... during the mid to late 1990's I was doing over a million dollars in sales (it wasn't all profit) working 7 days a week, probably 120 hours a week, traveling in and out of the country 4-6 months a year (hotels and airplanes were away of life) gave my family all the toys of a good income could provide (vacation home, land at a lake, boats, motorcycles, etc). When it crashed and burned (2003-05) I went from over a million dollars in sales to less than $100k ended up selling the toys and properties to try to stay afloat, basically lost everything including my credit - THEN the IRS came a-calling and demanded a LOT of money, but that's another story. We didn't declare bankruptcy, although we probably should have. No one to blame but me, I didn't see the market changing nor the new players using my business model better than I was.
The good years were great, the bad years were very painful - welcome to being a small business owner 