Is my insurance valid if I had a passenger without a seatbelt during an accident?

It depends a lot on what your policy says and the laws in your autonomous community, but most insurance companies can't reject coverage just because the passenger wasn't wearing a seatbelt, since that's the passenger's responsibility, not yours as the driver. What they could do is reduce the passenger's compensation if it's proven that the seatbelt would have prevented or minimized the injuries, but that affects the passenger, not your coverage. I'd recommend reviewing your policy in detail or contacting your insurance company directly to have them explain exactly what they're basing their rejection on, because it sounds like you need specific legal advice on this.