Assuming here that you have multiple routes at SITE A that can reach the SITE B destination IPs, or perhaps just a single summary route. There's always going to be a priority assigned in that scenario and it seems that .226/27 is winning.
The proposed solution is to apply a next-hop-interface static route at SITE A that forces traffic destined for SITE B (157.x.x.229/32) to go out the 12.x.x.229/27 interface. The inverse would also need to exist such that SITE B traffic destined for SITE A (12.x.x.229/32) would need to take next-hop-interface of 157.x.x.229/27.
Once that's in place, the IKE's should actually flow along the desired x.x.x.229/27 paths to establish the tunnel.