Translation:The siblings are walking on the beach.
Τα αδέλφια περπατούν στην παραλία means that the siblings are walking on the beach(on the sand). The siblings walk to the beach means that they walk to get there(to the beach) which is not the same. If you want to say "the siblings walk to the beach" in Greek is "τα αδέλφια περπατούν προς την παραλία"
I think it's pretty common to use the word siblings in English. Especially if the siblings are not of the same gender. In case they are, it's more common to say Brothers/Sisters. But that clearly depends on context, and there's none here. Siblings is perfectly correct.