The phonem [ĩ] is represented by the graphems -im / -in. (That doesn't exist in French either. The graphem in / em is not prounced as a nasal i in French.)
IPA presentation
graphem
example
[ĩ]
im
sim (yes), impedir (to impede)
,
[ĩ]
in
inseto (insect)
If one didn't pay attention perhaps one would not realise that it is nasale vowel. However there is a difference to a "normal" i.