See topic 21 on coding and compression.
Smaller frequencies are further away from the root.
The 2 smallest frequencies are siblings. This would then fail the definition of the Huffman tree. In this case, we would have to swap the leaf with a smaller frequency to be siblings with the other smaller frequency leaf. Also, by a counter example we can prove that every internal node must have 2 children to be a Huffman tree. Therefore, we need to reconstruct the tree by removing a leaf from the subtree of the node to make it its second child.