No experience myself, but a lot with members of the family.
Chemo is chemicals that are put into your body and individual bodies deal with that invasion differently.
When they say “this is the best” what they mean is “this combination of chemical has been shown to be the most effect in treating this particular cancer in this particular profile (age, sex etc.)
But if your body says no then they will try the next most effective. Often the difference in effectiveness is minimal. A slightly higher percentage of all clear or a even a slightly lower percentage of reaction, which your sister is unfortunately on the wrong side of.
It really doesn’t mean that there’s nothing else to be done which is perhaps what you fear.
It’s not a good comparison but I can’t have penicillin. Often it’s the most effective antibiotic but I have to have a different one and the infection takes a bit longer to clear up.
It may even be that they try the same drugs but individually as it might be just one or the fact that they are combined.
There are many, many different drugs used in putting together a chemo treatment and many alternatives which they can use.
I hope this reassures you.