I have started a relationship with my sister
Why on earth would you be in a relationship with your sister?
While I was in a trip with my sister, I felt being attracted to her and I couldn't hold myself.
Being romantically involved with family members is wrong. You both should seek professional help so you can discuss healthier ways to be siblings.
I think its morally alright in some cases. We are not committing any murder.
It's never morally okay in any case. You need to stop doing this right now, incest is horribly wrong.