For a school project, we each have to pretend that we are a person in the revolutionary war, and our classroom will be the wax museum. My person...

It’s hard to determine what is accurate information when you are dealing with facts gathered from long ago where perfect records may not have been kept.  I researched Mercy Otis Warren, as well, and got the same results you did. So, the best thing to do when you have contradicting facts is to look at the sources where you got your information.  Sources associated with government or public institutions such as colleges or universities are...

It’s hard to determine what is accurate information when you are dealing with facts gathered from long ago where perfect records may not have been kept.  I researched Mercy Otis Warren, as well, and got the same results you did. So, the best thing to do when you have contradicting facts is to look at the sources where you got your information.  Sources associated with government or public institutions such as colleges or universities are probably the most reliable because they often have access to important documents or scholarly research from which to draw information.  Therefore, sources with .org, .gov, and .edu are your best sites to choose from rather than .com sources. 

Remember, too, that historical records could differ because of the infant mortality rate at the time.  Many babies didn’t survive childbirth or their first few years of life due to disease and other health issues.  The mortality rate of the time could affect the historical accuracy of records kept at that time.  In addition, because Warren did not achieve fame until she was older and not a child, this could cause a discrepancy in the facts.  Brothers and sisters move away and get married making it harder to track down exact facts.

In addition, since several of the sources below say that she is one out of 13 children but differ in her place in the 13, I would go with the most consistent facts repeated by the sources, and that would be that she had 12 brothers and sisters. 

Here are a few sources I feel will be reliable information for your project.  They seem to be the most likely to be accurate.

https://www.gilderlehrman.org/history-by-era/creating-new-government/essays/righteous-revolution-mercy-otis-warren

(Library of Congress)

http://www.newworldencyclopedia.org/entry/Mercy_Otis_Warren

http://www.revolutionary-war.net/mercy-otis-warren.html