Today I called Sibelius.
I told this to my colleaque who passed my house when I was doing some painting, and I saw from his face what he thought about it.
Peter Sibelius answered in Lapinjärvi, same place where JS`s grandfather was from. He is a descendent of JS`s grandfather`s brother.
PS, b. 1957 in Tampere, is an arborist of the town Helsinki. He played tuba (!) in his youth in Lapinjärvi VPK torvisoittokunta (brassband) He and his wife Katri have 4 children, youngest is 2 years old Emil.
There is also an other family with name Sib, but they are not relatives. PS told that somebody, maybe who had been working for JS, though that because JS had no son, he will take that name and then we will have more people with this name.