Someone’s asked me this before and whilst I don’t feel qualified to answer I can’t help myself.
The Bible is not a handbook we refer to for every scientific or historical question. In other words it’s not the worlds book of facts.
The reason we don’t hear of other brothers and sisters of Cain is because the Bible is a story of Salvation. The biblical writers are only interested in giving us the narrative as it pertains to our salvation.
So just because the Bible doesn’t mention other siblings doesn’t mean there isn’t any other siblings. The writer assumes we are smart enough to conclude for ourselves that Adam and Eve had other children but himself sticks to the important timeline of the salvation story which is what the Bible is.
There are a few things we need to remind ourselves with once again. The Bible is not a reference for truthful and factual science or history but the truth of salvation and it only talks in this narrative as it always points to the lineage of Jesus and Our Salvation.