Thanks. Just an FYI for those suggesting the JGSI database - I actually searched it before making my post, but it's only of limited usefulness for my purposes here. The only records in the database that list Hebrew names (which hint at kinship) are for the small minority of members who are not buried in this shul's Waldheim cemetery (gate 40). Of the remainder (those in Waldheim gate 40), none of the rest with the name "Kaplan" appear to be in the immediate family of the Moshe Chaim Kaplan I mentioned. (Those include Rabbi Aryeh-Leib Kaplan, the longtime rabbi of the shul, who not only wasn't a close relative of the ones I'm researching, he most likely wasn't from Motol at all.)

Primarily I am trying to find:

-Any sisters of Shachne and Isaac. These are much trickier to find, since if they lived in Chicago, they would likely not have the Kaplan surname. (I already searched the non-gate 40 list for anyone whose father is Moshe Chaim. No dice - but again, the non-gate 40 names only cover a small minority of synagogue members.)
-Any brothers of Shachne/Isaac, who most likely lived elsewhere. That's why I asked about the early history of the synagogue - maybe there is an account somewhere that might hint at their other relatives.

A family member (whose entire maternal side comes from the Yanova/Drogichin area, very close to Motol) has a strong DNA match to one of Shachne Kaplan's descendants, and I'm trying to see if I can quickly identify others in this family among her top matches.

Adam Turner