Andromeda and Tonks have lost their loves to war. They are both in pain, they are uprooted and isolated in their grief. In the emptiness that remains the other is the only constant. It is natural to find solace and refuge in each other's arms.
Andromeda does something she really shouldn't but Nymphadora happily complies. Incest. Only suited for readers over 18.
This was the last time they both promised- each and every time. [incest]