Opera is full of siblings.  In just one work (Oh, all right, four really) we have:

Three Rheinmaidens
Alberich and Mime
Donner and Froh
Fasolt and Fafner
Siegmund and Sieglinde
Eight Valkyries
Three Norns
Gunter and Gutrune (who are half-siblings of Hagen)

And elsewhere in the Bayreuth canon we have Elsa and Gottfried, who admittedly makes most of his appearances in the guise of a swan (a mute role, of course)

Some more siblings (as they come to mind)

Miles and Flora
Auntie's Nieces (sort of)
Hansel und Gretel
Manon and Lescaut (as it were)
Fiordiligi and Dorabella
Barak, The One-Armed, the One-Eyed and the other chap
Marguerite and Valentin
Lord Walton and Sir George Walton
Norma's offspring (seen but not heard)
Elektra, Orestes and Chrysothemis
Cleopatra and Ptolemey

No 2 must be Manon Lescaut, at a guess - Puccini is a bit of a blind spot as far as I am concerned

pw, I assume your avatar is from the Glyndebourne Pelleas et Melisande, so I'm surprised you haven't mentioned Pelleas and Gollum Golaud.

On to Janacek - Steva and Laca  are step-brothers (Jenufa), Varvara is Tichon's foster-sister (Katya Kabanova), and then there are all those little fox cubs in Vixen.

Heading further west, Xenia and Fyodor (Boris Godunov).

It is from Boris - changed from Wotan to mark John Tomlinson's appearance at the Proms.

Still can't get No 1, and I'm still inclined to think it's Gluck...

And I can't remember whether the three noble orphans in Rosenkavalier are actually from the same family .

A quick glance at Kobbe reminds me that Barak's third brother was The Hunchback.  And that the Countess' brother in Capriccio ("Wo ist mein Bruder?") is, strangely enough, the Count, which doesn't seem quite right (but no stranger than the fact that the only decent tune in the whole work is given to the first horn.  I'll get me cloak).

And that tune was recycled from an old song cycle anyway... I have to admit to a liking for Capriccio though, especially the prelude. Strauss must have been a deeply weird individual to write such a thing when the world was being smashed to hell around him.

Yes, it is. However, I couldn't have picked an extract from Massenet's Manon as, for some strange reason, in that version Lescaut is Manon's cousin!


You'd be correct in that inclination...

Scary, isn't it!

Leonora and Carlo in Forza are barely on the stage at the same time. The only time they are, in Act II, she is in disguise as a pilgrim, he as a student.

In I Masnadieri, there are the brothers Carlo and Francesco, although they never have a scene together.

Nabucco starts with Abigaille and Fenena believing they are sisters, although Abigaille later finds out she was, in fact, a slave (which begs the question why/how she came to be regarded his eldest daughter?)

In Rossini's La Cenerentola, although Cinders is only a step-sister, there are sisters in Clorinda and Tisbe.

Moses and Aaron, of course, not only appear in Schoenberg, but in Rossini’s Mosè in Egitto, and Moïse et Pharaon, ou Le passage de la Mer Rouge.

In Albeniz's Merlin, we have Arthur and his half-sister Morgan le Fay.

In Borodin's Prince Igor, Yaroslavna (Igor's wife) is Prince Galitsky's sister.

Going back to the previous round, we find Ophelie and Laërte in Thomas' Hamlet. Staying with Shakespeare, sort of, Wagner's Das Liebesverbot, based on Measure for Measure, has siblings Claudio and Isabella.

Can you think of an opera where a pair of characters pretend to be brother and sister?

Very good, although knowing my lack of knowledge of G&S, you'll not be surprised to learn YotG was not the one I was thinking of. An Italian opera buffa...