My grandfather Theodor Erlanger (related to a number of people in the "German" list) was a collector of antique Bavarian folk art or peasant art as it was then called. He lived in Munich, but his large collection was housed in the family's summer-house in Bad Tölz, which functioned as a sort of live-in museum. The collection was partially dispersed when he fled the Nazis, but many pieces remained at the summer-house and were recovered when the house was re-acquired after the war. Does this count as an "art collection" or are you only referring to wall art and table-top art?
My mother (the daughter of Theo, the folk art collector) considered that "collecting" was genetic in nature and that what distinguishes collecting from hoarding is that "It is not enough to merely collect the items; you must properly house and display the collection." There was another relative in the family (and i have not yet tracked down his name, but i hope to) who collected antique puppet-theaters from all over Europe, prior to 1920 or so. The story my mother told me was that he was on vacation in Spain (from Germany) and saw a street puppet theater performance and was so charmed by the artistry of it that he bought it outright. This led to his subsequent purchase of many puppet theaters from all over Europe. (Several dozen of them, according to my mother.) One of the puppet theaters was in my grandfather and grandmother's house, and it may have been a gift or a bequest to one or both them from their elder relative. (They were cousins, and so it could have come down through either line of the family.) It was called The Nuremburg Puppet Theater. My grandmother brought it to America when she relocated here during the Holocaust. She gave it to my aunt and she put on performances with it when i was very young, with the help of my mother and aunt. I am going to try to research this "puppet theater collector" ancestor. Trouble is, i am 67 years old and all the older generations are now dead...