If you take the cm threshold to 1, I match several small segments with both your brother and sister -- and they both have several segments I match separately.

The 5 cm segment with your brother does not match my two known Herr matches.

I did look at your tree and Herr seems to be the obvious ancestral line, but you also have Dickson in Scotland. My Dickson family was in North America prior to the Rev War and probably came through Ireland.