If it feels good do it. If both siblings desire one another, then it's okay. Nothing is to be ashamed about as this should be a private matter anyway, unless they both desire it to be made known. Sibling sex is great. I know. My sister loves it and it's she who comes over for sex often.
If consenting between the two, okay. If both Parties know exactly what their doing and the age is legal on both sides, then they should be "allowed" to do as they please. Society's Taboo's are yesterday's wrongs. The two lovers will enjoy themselves as they should, No one should control another's sex life. Just do it !