Harrison is a GORGEOUS name which manages to be both classy and sweet and works well for any age!! I love all 3 of your MN combinations but my favourite two are Harrison James and Harrison Alexander, depending on which would sound better with your Surname. Of your listed Sibling choices I think the two that would work the best with Harrison James are Amelia Joy and Theodore Edward, and the two that would work the best with Harrison Alexander are Amelia Adelaide and Theodore Arthur. Harry, Millie and Teddy would be gorgeous nicknames for Siblings!! If I could choose any Siblings' names to go with Harrison James and Harrison Alexander then I would choose either those or the following:
Harry, Susy and Sandy
Madeleine Alice or Josephine Adelaide
The other Sibling names I love from your lists are:
Hope this helps!