I'm really sorry to hear your sister isn't responding to her medication as well as had been hoped.
Mutations and CML can be quite complex. The one patients know most about is called "T315i" which in the table below is the one which is in red (i.e. bad) for all TKIs except ponatinib. But, you'll see there are also mutations which nilotinib is not so good at dealing with that other TKIs are. So it's not quite as simple as one TKI being "better" than the other.
So whilst in theory there could be another TKI that could work better for your sister, am I right in saying that they have done a mutation analysis on her bone marrow? If so, those "cytogenetic" tests can see (or not) any mutations so you don't need to guess by trying another TKI.
Unfortunately, it does seem that some people who don't show up as having mutations don't do so well on TKIs. I don't know if this is because of an undiscovered mutation, or another reason altogether which is not yet understood.
In these cases, transplant is a viable option. There are patients on this group who have had a transplant and are doing really well, and gone on to life a totally normal life freed from the daily management of CML. But it is not always so simple as it depends on the quality of the donor bone marrow and many other factors. A transplant should only be considered as a last resort as it is a very serious medical procedure. I have seen a few cases recently where the patient has taken ponatinib for a while, as it is the most potent TKI, to prepare for a planned transplant.