I'm writing a fic on Isla, Phineas' sister, and I've been researching their birth dates.
Now, I've found two sources. The first one is a drawing of the Black family tree with information taken from a tree drawn by JK, and in says that:
First was Sirius, b.1845 d.1853, then Phineas Nigellus b.1847, afterwards Elladora,b.1850 and last Isla, with no date of birth.
The second source I found is the Harry Potter Wiki, which claims that Isla was born between Phineas and Elladora, instead of being the youngest child. No date of birth is given.
I admit that Isla being next to Phineas serves my story best, but I would like to be certain. If you are sure that one of the above-mentioned theories is correct, or another altogether is accurate, please let me know!