The surest way to avoid family friction is to talk everything out ahead of time. As difficult as it might be, it's healthy for everyone to know what your intentions are and for everyone to know in advance what they are going to inherit. The time to hash out inheritance issues is when you are alive and can make changes in your will if they seem appropriate. Get appraisals of all valuables. Get documentation and corroboration and back-up for any tax positions you may be taking that might be challenged after your death.
Family businesses are tough to value.
Be sure to get a detailed appraisal by a professional. Real estate requires a certified appraisal that considers comparable properties. Fine art is often reviewed by an expert panel at the IRS, so get an appraisal that is real. Review the decedent's homeowner's insurance floater or rider that lists artwork that is insured. If you're the executor, when you file the estate tax return, attach as much detail as you can provide supporting the deductions and claims and values that you have used.