This is the 5912th most frequent Russian word.
родственником
"Relative" or "family member."
Here, 'родственником' is used in the instrumental case to indicate the state or role of being a relative.
Он является моим дальним родственником.
He is my distant relative.
In this context, 'родственников' (plural of 'родственник') is in the genitive plural form, used to denote the object of search.
Искать родственников в архивных документах достаточно сложно.
Finding relatives in archival documents is quite challenging.
'Родственником' here is employed in the instrumental case to describe the person with whom the meeting took place.