This is my wife's company with your sister, and while I have two companies INC under one name, I feel the LLC is better for them.
Well INC you get double taxation, and they both offer limited liability, which means if someone wants to sue you then they can only go after company assets, We only have 9 employees, I do not think it would be worth it. INC works well with larger companies and also allows you to go publicly traded.
Really need to find an accountant and ask them.
I do know that the company I work for switch from LLC to INC because of how the taxes are figured. While INC has a higher tax %, what is taxable is less than LLC, and I believe you have more freedom to write business expenses off your taxes.