Is this fic finished? I seem to recall one similar here, not yet finished... But Seshomaru doesn't find out till years later that he had a son with kagome. I'd have to dig through my favorites to find it.
edit:
Here we go this is the one that it sounds like
When It Rains
www.dokuga.com/fanfiction/story/8716/1
Kagome Higurashi gets tracked down by a demon lawyer. He comes bearing the unfortunate news that her son is his lord’s heir. Fortunately, she gets the pleasure of meeting her child's father, and all of the drama that comes along with him as well.
Rating: MA ?-? Universe: Alternate ?-? Status: Incomplete ?-? Category: Oneshot Collections ?-? Created: 12 Aug 2013 ?-? Updated: 13 Mar 2014
Genre: Drama, Friendship, Humor, Kink ?-? Chapters: 34 ?-? Reviews: 130 ?-? Words: 28,297 ?-? Reads: 103,318