I agree that at this point her m-i-l is still that, at least until your wedding at which point the problem is (almost) the same as divorced/remarried.
Quote:

As she introduces me to her kid's relatives, ...


If I have understood the relationship you are describing here, "this is my child's paternal uncle, aunt, second-cousin, ..." Many of the fiance's kid's relatives on the father's side were no relation to her anyway, i.e. there is no second-cousin-once-removed-in-law.

<off-topic and silly>
By your method of typing fiance', shouldn't the possessive be fiance''s? It could be carried to extremes though: "On my re'sume', it says I pick pin~on nuts while whistling "Fu..r Elise" under a fac;ade."