This stone, weighing 1kg, is of great value.
=> The stone is valuable, no matter how much it weighs.
 The stone weighing 1kg is of great value.
=> The stone is valuable because it weighs 1kg.
 The stone - the one weighing 1kg - is of great value.
=> Mid-way through the sentence, the speaker inserts an afterthought. It's valuable because it weighs 1kg.
 I have 3 sisters, all of whom are living in abc town.
=> In that context, 'whom' functions as the object of the preposition 'of'.
Note, many speakers today omit -m on 'whom',
EX: all of who are . . . (how speakers use language)
EX: all of whom are . . . (how language works)