Gave them my blessing

My daughter is a few weeks from turning 15,I been noticing a pattern when my 20yo nephew turned up she would be all over him with hardly anything on,I asked them both out straight if they had feelings for each other to which they both said yes,when my daughter wants something she usually gets it so no point in fighting her,they never stop s******* hear them every night,he even walks around the house naked and i can see why she's a moaner.