Several years from now, this chocolate will be "ついでの" chocolate, though.
ついでのチョコ roughly means "chocolate prepared since she was preparing chocolate for someone else", "chocolate not made for the primary purpose", etc. This sentence means that, several years later, the girl would be preparing chocolate mainly for someone else whom she loves, and preparing chocolate for her elder sister only as the secondary purpose.
See: Difference between ついでに and たびに in the following example
In Japanese Valentine's day culture, there is something called 本命チョコ and 義理チョコ.
Valentine's Day, Japan: A How-to Guide
Giri-Chocolate means “obligatory chocolate.” This kind of chocolate refers to the chocolate you have to give to people (who aren’t really people you love). These could be people like bosses, coworkers, male friends, etc.
Honmei-Chocolate means “favorite chocolate.” This kind of chocolate is the kind of chocolate you give to the one you want to express your love to. These chocolates tend to be more expensive or possibly even home made.
So the friend is saying the girls is making 本命チョコ for her sister for now, but it would be 義理チョコ in the near future.