mother in law stays with us. where do i post her 1099?

You MIL's income does not go on your tax return.   And...if she had more than $5050 of income --not counting Social Security---she cannot be claimed as a dependent.   What kind of 1099 does she have?   There are over a dozen kinds of 1099's.    She might need to file a tax return for herself.  

You don't report her income on your return even if you are claiming her as a dependent. 

If she received form SSA-1099 , she would report it on her own return, but only if she makes enough income to be required to file. If Social Security is your dependent's only income, most likely she doesn't need to file a return.

@adair-alonzo the simple answer is that she 1099 goes on her tax return - not yours'.