W: Wish

If you liked Onegai Twins I recommend watching this next. It's alot like Onegai Twins due to two sisters trying to be with their brother. (not in the weird incest way that some think >.<)  

Both are stories about twin brothers moving into a new house after the lost of their parents and the female main character seems to love her brother more than normal. And both are the harem type.  

These both start out as your typical 2004 harem anime, being all slice-of-life and happy and oh, look, a love triangle, how creative. However, all the sudden, BLAM supernatural. Then the show gets much darker.  

Incest romance with drama that can make you cry for some random reason 

Still Incest romance but not really romance story in D.C Da Capo II 

both series contain memory loss as a plot point and the conflict created when the old memories are returning 

Both: based on a VN, protagonist has memory loss, and "something is not right with this world" is pondered a lot. It's hard to give more information without spoiling both series but VN players of Little Busters! and fans of Key's works will understand the reoccurring theme. And that theme is in W: Wish. 

in both series, the male leads juuna tono and ryoh mizuhara lives with their respective younger sisters senna and shizuka as their respective parents died before the series began 

W Wish is one of that anime series based in a date game, like Memories Off. The first is more harem-like style and during some moments you'll find it very boring, but in the final sequel of episodes the storyline get better. The OP and ED themes are great, and overall is a anime worth watching. It didn't have a sophisticated development like Memories Off, but surely you'll enjoy it. Ah, it has girls in swinsuits, if you're a ecchi fan.  

Some later events in W Wish made me remember Gift for a reason.... (which i will not spoil here) Gift is also a School/Romance/Drama Title with similar story 

Similar slice of life/school/harem type stories, both containing similar characters in similar situations, and the school looks pretty much exactly the same in both of them. 

In both anime the main character has lost his parents and his memory when he was younger, but starts remembering things when he meets a strange girl who seems to know him. 

The worlds are similar