I absolutely HATE a thief...

Ok SO my sons birthday party was yesterday. We had a blast a total of 55 people came! Today my mom find out while the party was going on in the front yard someone stole her brand new leaf blower from the back. We narrowed it down and it couldn't only been one person (my cousin) anyways I'm fucking LIVID! Our own family to steal at my son's PARTY?! My mom is very upset and hurt she was almost in tears. I'm my moms only child and in very protective over her. What should I do? 
A) go over their house at night and search their car & take it back. (I KNOW it has to be in there)
B) confront him (I will cuss him out)
C) tell the police (if so how do I go about reporting?)