Thanks for the suggestions, Gary!
I'm working on baptisms for the Fischer children; I've written to St. John's (the mostly likely church) and hopefully the Pastor will respond. Marriage witnesses haven't panned out - neither couple had family members as witnesses.
I have Louisa Fischer's immigration records. She arrived in 1867 with her family. Unfortunately, the Fischer men have much too common names to be able to positively identify them in the immigration records.
I had forgotten about checking cemetery records. Thanks for that reminder! The info about "T" vs "F" is useful too - I have a few censuses where that transcription error was made.
Thanks again for the tips. Hopefully one of them will be the break I need.