Just want to point out this out again.
Notice who says Ni and Si are generally similar => mostly Ne dominant (ENTP, ENFP, INTP)
Notice who say Ni are more dissimilar to Si => Ni dominant (INTJ, INFJ, ISTP)
The pattern says a lot to me...
It suggests that unless you're an active user of a introverted perceiving function, you cannot truly know how it works. And even if your dominant function is Pi, you are inherently weakest with the opposing Pi.
For example, I'm an off the chart Ni dom, I KNOW I rarely think about events of the past, or be able to recall sequential details of any sort. Because of this lack, how can I accurately compare the Ni experience with an Si one? Even if I ask an ISXJ for their thought process, I'm filtering what they said through the mere act of listening.
So really we can only go by the definition or description provided by Jung as what Ni and Si are. Both are internally reflective, information gathering processes but that's all the similarities they share.