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Do you have a copy the will? Most of the time (but not all) an 19th century and earlier will specifies relationships. Generally the wife is first, male chldren are next in order of birth, then the females in order of birth, then other relatives and friends (as in "my good friend John Doe"). Often a son is named executor, sometimes with the wife as co-executor.
If this is not the case, you need to examine all the probate records of the estate including guardianship bonds (very important if minor children are involved), executor bonds if any are required, inventory records, estate sales, and final reports. Some of these may not be found for persons who died testate, but should be if the person died intestate. Look at deeds made, or recorded by the testor. If the deceased was alive after 1850, censuses are a way to determine relationships, and especially from 1880 forward.