Can't add a tiledbackground object into family

I just noticed that the tiledbackground object not show up in the family list. Is it a bug or its impossible to put them into the family with other sprite objects family?

families are object specific. You can have sprite family, tiledBG family... but you can't mix different objects in one family

See Families in the manual. A family must consist of all of the same kind of object (e.g. all sprites, all tiled background, etc.). So you can create a family of tiled backgrounds, as long as all the other objects in the family are tiled backgrounds.