Congrats JBDad!!! Girls are so much fun and she's lucky to have 2 big brothers! Don't be surprised if she seems into their stuff more than things geared to girls. My DD is 2.5 yrs younger than DS and she loved all toys for boy until about 4.5y. At 18m she could name all the Thomas trains and at 2y, built her first items with Legos! She's definitely more into both girl and boy stuff now but will still play with whatever her big brother is in playing with.
Another website:
AOPS resources Several intro to algebra books, etc. Might be more appropriate in a few more years but at the rate that your little guy is going, sooner rather than later seems more likely!
Good luck and get some sleep!
