Can we use “ti amo” for expressing love between siblings?
And we use “ti voglio bene” for expressing love between two passionate lovers?
Even though the verb amare is supposed to cover all meanings implied by the English verb to love, this is not how it is currently used. You would still say like this:
Ama il prossimo tuo come te stesso.
You shall love your neighbour as yourself.
You would start to sound slightly awkward, but still not necessarily incestuous, with a sentence like the following:
Amo mia sorella.
I love my sister.
But the serious problem arises with ti amo. This is a fixed way of saying I love you in a romantic sense, notwithstanding a long tradition of poor translations in movies. There are teens using the sentence in a "creative" way, like ti amo come amico or ti amo di bene, but it's more a way of being silly or overly sentimental or it's just a way of mocking wanna-be boyfriends being "friendzoned".
If you care for somebody in a non-romantic sense, go for the standard
Ti voglio bene.
I care for you.
This can be used in a romantic relationship, in order to highlight the more altruistic, tender (perhaps... extra-sexual?) side of the feeling felt for the other person.
Ti amo, ma ti voglio anche molto bene.
I'm in love with you, but I also care for you.
Actually, you could contrast amore e voler bene:
"Ti voglio bene ma non ti amo più", un libro di Andrew G. Marshall
"I Love You But I'm Not in Love with You", a book by Andrew G. Marshall
Don't be misled by the last example, though: to be in love is not always translated as amare, because the Italian language often uses essere innamorato as opposed to amare to express the nuanced difference between being in love and loving. The point here is that the underlying implication of using innamoramento vs amore by many speakers is that there are two distinct phases of romantic love: the early, exhilarating stage and the subsequent developments. I don't believe that this distinction is as popular in English-speaking cultures where you are expected to fall in love and then stay... down there. Actually, the two-stage theory has supporters everywhere but not all languages have it embedded like Italian.
Yes, it's possible to use "TI AMO" between siblings but normally it's used between lovers.
There isn't a specific environment where use only "TI AMO" or only "TI VOGLIO BENE". Maybe, it's possible to tell that I use "TI VOGLIO BENE" when I'd like put in evidence that I want give you something and I hope that you are fine. More times it's used by parents for their children, it's used between friends, ... Using "TI VOGLIO BENE" there is one I and one you but it's not necessary that there is one we.
I use "TI AMO" when I'd like to put in evidence that I need you to be fine and of course I want that you are fine to so that I'm fine. Using "TI AMO" it's how there isn't one I and one you but one we.
"TI AMO" has a strong meaning. It's used between lovers, but there are many forms of love. A parent can say "TI AMO" to its child. When we say "TI AMO" it means the we are strongly linked to that person, we care care about he/she as ourself or more. "TI VOGLIO BENE" has no romantic meaning. It's less strong than "TI AMO", even if emotions are always involved. It means: I care about you and I wish you all the best.
It's possible to use "TI AMO" for siblings, but it will sound very strange. It's used for lovers, or between a child and a parent, or for other few special occasions ("Gesù ti ama", "Jesus loves you")
"TI AMO" is a very strong statement: Instead, "TI VOGLIO BENE" could be use for a frendship relations, and no one will ever think that the people who said you "TI VOGLIO BENE" il your lover... but if somen