He was compared to River after River's death because they look similar and were both talented young actors. I don't think anyone knew of Joaquin when River was alive either so it's not a case of him being the lesser brother. Joaquin found fame early and his siblings found it later. River was definitely a talented young actor but who knows how he would've turned out as an adult. He was more a peer of Johnny Depp's than Leo though. Him and Johnny were very close friends.