This is not how schools determine which schools are considered “peer schools”. Plus…some schools have much deeper pockets for merit aid and award it to more students.
As noted above, to ask for reconsideration when another merit offer is higher, the schools need to be very very similar in terms of all their demographics. Also, you should look at the criteria for receiving merit aid.
In other words…if school A offers merit aid typically in higher amounts to students with perhaps lower stats, school B (your $15,000 merit school) might not consider school A to be a peer school in terms of how aid is awarded.
Also, if a college has published criteria and stats for merit awards, they might not budge at all.
And just an alert…there are some colleges who won’t even discuss this with you. They give their awards and that’s it!
BUT if you don’t ask, you won’t get…so it’s worth a try.