Yes, they are siblings. Mum laid four eggs (first breeding session this year having been part of a bonded pair for 2 years) and only two eggs were fertile.
I'm not a breeder; this is a result of my lovely 4 year old Yoshi (male) being brought from our breeder as a baby and him being paired with an old lady (Linnie) for quite a while. She died (of old age, I suspect given the rings around her eyes) about 2 years ago, and he called for her for a long time afterwards, so another mate was sought for him. 2 years later the new pair have mated!
Yoshi has proved to be a fantastic dad. He's a lovely little chap and I've always felt very close to him.
I'm very lucky in that the lady I bought him from (nearly 5 years ago) is still there for me to hand out some advice. She doesn't keep Linnies anymore so I try not to hassle her too much.
The birds actually belong to my 16 year old daughter, and she hasn't decided what to do as yet. I am rather hoping my breeder will take on one of the youngsters and pick up where she left off a few years ago: she is a fabulous lady - very, very knowledgeable about Linnies and a total whiz at making handmade toys for this breed of birds.