My brother in law left his lawn mower at my house for six months can I claim it now?

Why would you want to, don't you have one ?

No, you would have to inform him, in writing, that he has a deadline to remove it or you will consider it abandoned property. If he doesn't meet the deadline, then you can lay claim to it.

Agree, have you notifed him that it is still there, have you asked him to come get it.