How to handle young adult suspected of stealing cigirets and money?

I have a good suspicion that my little brother in law he is 15 has been steeling money from my wifes purse and smokes when ever he comes over I have seen him taking cigirets told him to stop he said ok. Now when I ask him about more money and smokes missing he always says " I sware to god I didn't take them" what I am asking is he is a good kid most of the time do I ban him from my home? Any ideas would help.