It’s basically an ugly setup that doesn’t have a clean answer.
Depending on what you want to access, you could potentially NAT, with specific routes for like /32 on IPs for specific services you need to answer. But if there are any collisions, it won’t work. It will be particularly troublesome if SITEA needs to access SITEB through the S2S.
Frankly the easiest way is to re-ip whichever one will give you the least amount of headache.