Not that I think it is fair but the source is probably from Rachel and Leah. When Laven switched them and Yackov asks why Laven answers “lo yaasa ken bmkominu” it is not done in our community to marry off the younger before the elder.
Is there someone your older daughter is close with that she can discuss this with? Someone who is objective? perhaps someone for her seminary in Isreal?
As to Emuna, If you have emuna perhaps this is not the shidduch for the younger one.