I suspect my nephews of theft. How do I talk to my brother about this?

Elswhere I described the theft of about $7000 worth of kayaks and gear. The extent of the theft of my outdoor back country and climbing kit has yet to be identified. I still need to inventory everything.

That the thieves knew what to take and what was stored in the shed points to the fact it was someone close. It would have been a coordinated effort to load everything onto and into multiple vehicles (they also stole a canoe). They also would have known my brother was not currently residing in the house and that the property was largely unattended. My thoughts then keep turning to the inclination to think at least one nephew and their friends are responsible.

Do I bite my lip, ignore my suspicions, and wait for the insurance to be worked out? Or should I be proactive and let my brother know my thoughts.