Is it permissible to marry the daughter of a maternal uncle, if the elder sister was suckled by the younger maternal uncle?

Nursing establishes the same prohibitions as lineage. Thus, the nursed child becomes the son of the woman and her husband, the owner of the milk. This applies exclusively to the nursed child and not to their siblings. Therefore, your elder sister being nursed by your maternal uncle does not make his daughter unlawful for you, unless there is direct nursing established between you. It should also be noted that for nursing to be prohibitory, it must consist of five distinct feedings.