So I think of true harem meaning more like The Last Sovereign where all the girls are together and aware of each other and the fact they are all sleeping with the same guy. Most of the ones I know are more like DMD where you just sleep around with a bunch of different girls.
Melody - Same dev as DMD. Not harem but you can get with multiple girls along the way and end up multiple in one of the endings. No incest here.
Thirsty For My Guest - Main girl is ex-girlfriends daughter so not technically incest. Not all the girls are younger but several are.
Dr. Amana, Sexual Therapist - Stepdaughter is one of the girls but you can choose not to pursue her that way.
Something to Write About: The Author - You're not actually that much older than the girls but enough that it comes up.
Ecchi Sensei - A good bit of incest but also lots of other young girls.
What's Mine is Yours - Build a harem of your students at your wife's behest!
To Be a King - I actually just found this so I haven't played it and can't really vouch for it but it looks promising.
Sugar Baby Galore - The girls are slowly learning about each other as you progress with them all.
The Manifest: Shadows over Manston - Your stepdaughter, her friend, and a few of their classmates are available