I have the Brookings County Census records for 1860,1870,1880 and there are no Den Otter Family in there.
I also have Brookings County History Book that has many family histories in there. There are 2 Den Otter families listed one his Bert and Florence they lived at 124 west 4th street in Volga. There is also a picture of them on there porch with their dog Timmy. The history is about 3/4 of a page.
The other is Gerrit and Viola Helen (GRANUM). Violia was a twin. This family history is about a 1/4 page.
Let me now if you need more info