@Dzechy Welcome to Atheist Repubic.
As long as one does not harm others and both (possibly more) parties consent, then it is none of my business. What people choose to do in the privacy of their bedrooms is theirs and theirs alone.
What moral aspect? From one perspective it may be repulsive, from another the interaction between siblings or a parent and child is an expression of love. Who gets to choose? Who gets to play god?