Kono Naka ni Hitori, Imouto ga Iru!

They're both traditional harem series where the MC is from wealthy and powerful family. Nisekoi is less ecchi than KNHII. 

In a harem comedy anime, one major problem writers have to overcome is how to maintain the "harem" situation, since in normal circumstances any such arrangement would quickly collapse. Most harem comedies do this by making the protagonist or the girls (or both) defective in some way (stupid, socially inept, etc.). One of the reasons I like Nisekoi is that (while some of this is still present), there are more complex plot devices at work which which sustain the harem (the question of the childhood promise and the need to fake a relationship to avoid a turf war). Kono Naka does this even better; the protagonist is even more sane but is dealing with the twin pressures of having to marry before graduating highschool and knowing that his biological sister (whose identity he doesn't know) is a student at the school and actively trying to seduce him. The circumstances and decisions of the protagonist are things a reasonable person might do, and this makes for a much more interesting story than "the protagonist is too emotionally immature to choose a single girl". read more 

Harem shows, but with a little added mystery. Our main bro has to uncover the secret one of the girls has. In HenSuki its the person behind the love letter, NakaImo its one of the girls being his sister. 

A better version of Nakaimo. Extra pervy though if that bothers you. (each of the main girls has a specific fetish) 

These two series are both about love between a brother and a sister. In NakaImo, the main guy must find a marriage partner at his new school in order to claim his huge inheritance. The catch? His sister also attends that school. In OniAi, the main guy is reunited with his little sister after six years apart, only for said sister to repeatedly express her incestuous love towards him. NakaImo has a little more mystery (nothing complex at all) and OniAi is maybe more comedic, but essentially, if you liked one, you will enjoy the other. 

OniAi and NakaImo the younger sister is DEEPLY inlove with their older brother OniAi and NakaImo is a School, Romance, Harem, and Comedy Genres it makes you want to watch more 

Both series contain a similar story and general feeling as the main protagonist becomes a figure of importance at school. Both series are categorized as harem as various girls competes for his affections. And of course, the girls have various personalities that creates comedy, drama, and romance.  

The MC from both series came from a rich ass family and as soon as they transfer into a new school the all the Female MC just......you know harem stuff 

- similar main character - brother x sister love - harem - similar story - transfer student, who is going to get married  

Similarities: Both stories are harem romantic comedies with harems with a brother x sister relationship at the center. Differences: In Oreimo, the sister is an otaku. In NAKAIMO, the brother doesn’t know who his sister is. NAKAIMO is much more ecchi. NAKAIMO is one season and one OVA, whereas Oreimo is two seasons and six OVAs 

Both series contains a main protagonist who has several girls of various personalities interested in him, with one of them being a sibling in the series. Both series contains a school like setting with comedy, drama, and romance. Both series also a storyline that gets more complex as the episodes progresses with twists and angles. 

male mc falling in love with girls that might be his twin sister 

The story is similiar because MC made a promise to his childhood friend to marry and the another one is made from family. But these anime didn't similiar very much