The anomaly is your sister's 68 cM in 6 segments with your 2C as it appears too low.
Since you're on Ancestry, there's not a lot you can do to figure this out. Try comparing the sets of Shared Matches of you versus 2C, your sister vs 2C, you versus 2C1R, and your sister versus 2C1R. See if that sheds any light on the problem.
This resolved itself today when I had email contact with my second cousin's son for the first time, and he told me that I had recorded him as being the offspring of the wrong one of three sisters in that family.
Once I switch in the correct lady, and call her 2C instead:
I have 106 centimorgans shared across 5 DNA segments with 2C, and 104 centimorgans shared across 6 DNA segments with 2C's son 2C1R
my sister has 218 centimorgans shared across 10 DNA segments with 2C, and 94 centimorgans shared across 6 DNA segments with 2C's son 2C1R