I'm trying to understand Islam's stance if a father denies a marriage proposal without proper Islamic grounds, favoring a cousin instead

As-salamu alaykum, I was hoping for some clarity on a situation. Let's say there's a brother and sister who have completed their studies and feel ready for marriage. They share mutual respect and affection and are financially prepared. However, the sister's father is rejecting the proposal without any valid Islamic reason-simply because he prefers her to marry a cousin from back home. Both families are from the same background, so that's not an issue. What does Islam say in such cases? Is it possible for them to proceed with a nikkah if the wali (guardian) is unjustly withholding consent? Could an imam act as a wali? They are considering their options, as the delay is causing distress, but they want to ensure everything is halal and pleasing to Allah.