Is it permissible to marry the daughter of a maternal uncle from his second wife, knowing that the elder sister was suckled by his first wife, and the number of sucklings is unknown? And what are the limits of impermissibility in this case?

Suckling establishes the same prohibitions as lineage. Whoever suckles from a woman becomes a child to her and to her husband, the owner of the milk. This applies exclusively to the suckled individual, not to their siblings. Based on this, your older sister suckling from your maternal uncle's wife does not prevent you from marrying your maternal uncle's daughter, whether she is from the suckling wife or from his second wife, unless a valid suckling relationship is established between you. Doubt regarding the number of sucklings does not establish prohibition, and for prohibition to be established, there must be five full, satisfying sucklings.