City directories are available on Footnote for both Brooklyn (starting in 1862) and New York City (from 1786). Brooklyn addresses can also appear on the NYC directories. Footnote is available at most Church of Latter Day Saints Family History Centers and you can also purchase a subscription for one month at a low cost. Ancestry has quite a few years available but not the 1860's range you're looking for. Another possibility would be to search the Brooklyn Daily Eagle to see if you can find mention of Nellie. It is available online at the fultonhistory.com site. It sounds like you don't have the names of your gr-grandparents from your grandfather's death certificate? If you had that, you could pay for a search through the Roscommon Genealogy Centre. I always thought I didn't have enough information, but I was able to get some great results from the Cavan Genealogy Centre, and it was well worth the fee. Without the gr-grandparent names and with the siblings living/buried in two separate cities, it would be hard to match a Brooklyn Higgins to your grandfather. Is there any possibility of someone in your family having cards or letters belonging to your grandparents? Or a listing of gifts or guests at your father's wedding? Usually godparents would be a good clue but in your case, I doubt someone would have made the trip from New York to Illinois. Good luck! Melanie Orlando