I have been having cyst for the past 11 years and they are super painful, so I know
how your sister feels. My last cyst got as
big as 6cm and the dr didn't do anything. Thankfully it's shranking and the last US I had it was at 3.6, not really sure what type of cyst your sister has, but my dr told me to hang on and hopefully it goes away on it's own. Mine usually repture on their own, but this one was different. It just keep on getting bigger and bigger.