Greeting, Alan Lewis - welcome to the Urth List.
Here are some possible reasons why Nettle would want to return with Silk/Horn to the Whorl:
1) She has always admired Silk and would want to follow him as both a friend and "disciple".
2) Silk retains in his personality what little remains of her beloved husband Horn.
3) She now knows that her husband Horn is dead. She is a widow, and wants a new start. There's no point in staying on at the mill waiting for Horn to return. Better to let her children take over and run things. Give them a clear run and get out of their way.
But you're right to comment on the oddness of the book's ending. I have to confess that I find many details of the ending of RttW odd and unsettling. I need to raise the business of the killing of the inhumi at the wedding again some time...but not now.
Nigel