Why is 'Чи' omitted here?
It is not ommited. This type of question is not common in colloquial speech. Questions with Чи belong to sort of "bookish" type (poetry, classical prose, formal document language (like surveys, polls). I advise not to report these.
I like it with the 'чи'. It sounds better and it makes more sense.
In spoken, can we omit 'ти'? Любиш грати на піаніно з сестрою?