I had emailed some churches looking for my grandparents' marriage certificate. Someone at one of the churches I wrote actually did some research for me – finding a bunch of things out I already knew, but it was really generous of them to do the searches. One things they found that I didn't have was my great-grandparents' obituaries from one of the Buffalo newspapers.
The one thing these list is names (including married names) of siblings. I had them for my great-grandmother's siblings, but I didn't have them all for my great-grandfather's siblings, and I wasn't sure how many siblings he even had. Turns out a couple I thought were his sisters must have been cousins. They immigrated with my great-grandparents, listing right before them on the Ellis Island manifest, but they were either his cousins or nieces or something.