Twincest is generally good, assuming both are female. However, as up2u stated, it can get awkward when other people are in close vicinity to you, leading to bad, bad things :P
Higurashi has arcs that bring up questions, then those that answer them.
For Example...
When we think that it was Mion who went on the killing rampage, but actually Shion - we then see from her point of view, which answers many many questions.
