Like many others, I watched this series after I finished Onegai Teacher. I came in with expectations for the same level of romance and involvement I had with Onegai Teacher, but I was surprised to find out that the two series were shockingly different.
The problem with the story is that it had a very weak hook. This is not a series where you see love develop like in Onegai Teacher. Instead, Miina and Karen's affections for Maiku are very blatant. Most of the story focuses on their conflict between being in love with Maiku as a brother and as a potential lover. This led to a ridiculous amount of misunderstandings and fanservice opportunities. It's great if you are looking for an ecchi harem that focuses on twincest, but in reality it lacks the depth and drama to really draw you in.
The story gets really repetitive at times and they seem to reiterate the same points over and over again like when they state "We might be relatives.... but we might also be strangers." Thoughts like that are so obvious that they did not need to be repeated more than once or at least could have been expressed in different ways. However, the show starts to pick up in the end so if you can bare the first few episodes you should be satisfied with the way the series ends.
Art and Sound
I liked the use of the split screens in some of the episodes as well as the slow motion running with the Chariots of Fire-like background music. However, I did not care for the OP and skipped it for the most part.
Although the two series share many of the same characters, it seems that they have different personalities and roles in each series. In Onegai Twins, Mizuho played a large supporting role as the meddling, ditzy teacher. That is in stark contrast to the more wife-centered romantic role she played in Onegai Teacher. However, it was still nice to see her involved in the anime and she does provide some fanservice. Other characters like Ichigo and Matagu took on larger roles in this series than they did in the previous one. Ichigo is the student body president who continues to meddle in the love affairs of the students and Matagu is a perverted older brother.
As for the new characters, I find them to be on the weak side. It's nice that Maiku isn't a weak, wimpy character but I didn't buy into his strong, silent type of character. He spent most of the series trying to hold back his feelings and that seemed to stop his character from developing. In addition, Miina and Karen weren't very attractive female leads in my opinion. They fell for him too easily and their histories were not very interesting. However, to be fair, the heart of the series is really how much all three of them care for each other and have become their own little family. In that respect, watching the bonds between these characters grow was an important and pleasant part to watch.
I think people will enjoy this anime if they are watching it for the right reasons and come in without high expectations. It's not a very dramatic, hard to follow, romantice series. Instead it is a light-hearted portrayal of a group of strangers who become family. There are a lot of ecchi moments and you can't go one episode without nudity. Also, sometimes the episodes are so stupid they are funny. So if you are just looking for something to pass the time then I think you will really enjoy this anime, but if you are looking for real romance then I would probably look elsewhere.
If you watched the first series, you can't go wrong watching Onegai Twins, but do not expect the same kind of story. However, if you are looking for easy watching with some feel good moments and a happy ending then I would recommend this anime.