How can I access resources in a peered VPC over site to site VPN?

The short answer is you can't because VPC peering doesn't support transitive routing. The rule for transitive routing is that the traffic must either originate or terminate on a network interface in the VPC. This is why VPC Peering isn't transitive - no network interface is involved, so no route table. Also this is why Transit Gateway is transitive as its attachment points in a VPC use network interfaces.

The usual approach is for your site-to-site VPN to have VIFs to each VPC, or better still, to use Transit Gateway.

Another workaround is using a proxy instance to put a network interface into that VPC you want to transit through. See for example the standard "Transit VPC" Strategy that was more commonly used before Transit Gateway came along.