Sounds a lot like
53x - Homecoming. The MC has a lot of ventures, one of them is becoming a photographer, another is a covert operative. The mother you're living with is actually your stepmother (and the other girls are your stepsisters), but you meet back up with your birth mother (who's also a covert operative, and was essential to MC's training as one), and you do have sex with her (like the "good old days" - as in, not exactly for the first time).
The tags for it in this thread's directory is correct, though. You live with your step family, but your also get together with your biological mother, and someone who's revealed to be your biological sister.
Sadly, it got abandoned, partly because of real life stuff (you can read through the thread for more info).