Tongan /s/ seems to be the regular reflex of *t before /i/. Wikipedia says
Tongan has retained the original proto-Polynesian *h, but has merged it with the original *s as /h/. (The /s/ found in modern Tongan derives from *t before high front vowels).
That article also says that as a general rule, Tongan retained a Proto-Polynesian phoneme reconstructed as *l but lost a different Proto-Polynesian phoneme reconstructed as *r. In fact, Tongan /l/ vs. ∅ is apparently the main criterion for reconstructing PP *l vs. *r, as they have merged reflexes in other Polynesian languages. So Tongan nima seems to be a real outlier, which is in agreement with the note on the Wiktionary page Reconstruction:Proto-Polynesian/rima:
If it were *rima, Tongan would be expected to have *ima, and if it were *lima, Tongan would be expected also to have *lima. No other Polynesian language fully distinguishes /ɾ/ and /l/. [The reconstruction *rima] assumes that nima evolved out of *ima, which may have been modified to avoid confusion with a common homophone.
Alternation between /n/ and /l/ or /n/ and /r/ (either sporadic, or as part of regular sound changes) is not that uncommon. There was a previous question about /n/~/l/ alternation in Asian languages.