With regard to siʻi (def. emot.), the book Intensive Course in Tongan explains its use better than I can:
Intensive Course in Tongan wrote:Siʻi immediately precedes the word of phrase it modifies. The emotion expressed by it may suggest tragic misfortune, pity, endearment, or humility.
ʻOku fakamamahi ʻaupito e siʻi mate e faʻeé mo e siʻi fānaú.
The unfortunate death of the mother and the poor little children is very sad.
Si'a (indef. emot.) is used similarly.
Tongan Grammar wrote:Kuo lavea si'a tamasi'i?
Has a child been hurt?