What is the integrity of your Sister in Law? Her character? Is she one to lie or tell stories? Does she have a history of inflating stories?
What is the integrity of your daughter? Her character? Is she one to lie or tell stories? Does she have a history of stealing?
Then, that will guide you on what to do next.
Encourage your daughter, calmly and in a comforting way... to tell her side of the story. There must be some kind of explanation. Then, tell her you love her no matter what and don't be scared to tell you...even if it is something negative about your Sister In Law, or herself.
Next, DOES your daughter even wear make-up? I imagine at 11 years old, a girl that age does not "need" make-up, much less to steal it....
The thing is... there must be a "reason" your Sister-In-Law said that about your daughter... whether true or not. Does your Sister in Law "like" your daughter? Do they get along? WAS your daughter even anywhere in her home or next to her purse, to where she 'could' have taken her make-up? Is there an alibi?
DID your Sister In Law ACTUALLY SEE your daughter stealing it? If not, it is just supposition. DOES your Sister In Law have any kids of her own, who could have taken it?
WHY does your Sister In Law think your daughter stole it? Did someone 'tattle' on her? Was it just hearsay? Or is she being blamed for no reason?
The thing is, a child needs to know their Parent "trusts" them... but so talk with her about it and see what the truth is, using discernment per both parties.
And, sometimes some people just won't admit to things. So it's hard to find out the truth.
If no one actually "saw" your daughter taking it....then it is hard to prove it, or to blame her.
Really decide what the motives are, and what their "character is... then one of them must be mistaken.
All the best,
Susan