It depends on the university course.
I wasn't allowed to take Add maths in sec school.
I was taught Engineering maths in poly, and the syllabus resembles
Add maths taught in sec school, except some parts are a bit harder,
while some parts are not as hard.
When i was in poly, i took a supplementary course on higher
engineering maths (it's a few topic from university maths) and
found that some parts of it requires some knowledge from additional
maths, but not totally.
In my opinion the maths still rely very much on the basics, it's
requires you to derive some equations, and prove here and there,
thus it requires good manipulation in the sec school e maths
part(factorisation, expansion) , as well as some advance knowledge
in A maths, which will help to speed things up and understand
concepts better.
Now for your question, i will suggest that your sister should
decide if taking add maths will really hinder her chances of
scoring well and get into poly.
If she will fail o levels or do very badly just because she has to
concentrate on add maths, then it's better to drop it, dun fail the
whole o level just because of one subject.
If she can cope with it, then it's a bonus to know A maths, it will
help quite a bit. Like when you see this equation, you know what to
do, cos u have done it before. It's something like once u see sin
30, u know the answer is 0.5, when you see a complicated mixed
fraction, you know how to break them up, u know how to use l
hopital rule, etc etc
I heard JC maths will help a lot more in uni.