A 33mm cervix is pretty good and not considered concerning at this point, the tricky thing with IC is that you can have really good length and things can change even over night which is why even weekly or biweekly scans don’t always work to catch IC. I had no risk factors and had my 20 week scan with a long and closed cervix and less than 10 days later I was dilated and needed and emergency cerclage. IC does have a genetic component but just because your sister has it doesn’t mean you will. Unfortunately it sounds like the only option os to continue to pay for private scans for the next few weeks if that will give you peace of mind. They won’t do a cerclage or anything after 24 weeks in most places so most would stop checking after that anyways. Just remember cervical length scans are user dependent so you can get variation in length just by the person scanning differently, and how your baby is positioned that day etc. so you may see variation in length just due to that. Good luck!
@Rosa do you think this measurement at 21 weeks is concerning