If you have O- blood, your risk was basically cut in half.
My sister has O negative and she got it. She was vaxxed and boosted.
I have B negative, had two jabs, no booster, never got it. I have several friends who were jabbed and boosted and got it twice.
Having a half-chance does not remove the risk entirely. There are still other health conditions or diet or her BMI to consider.
If my risk is otherwise 20%,but because I have O- blood I only have a 10% chance of getting it, it doesn’t mean I won’t get it. It just my risk is about 10%.
Your sister probably still had some protective measure from it getting worse than it was if she had another blood type.
https://www.verywellhealth.com/type-o-blood-covid-19-5074429
My son is 0- and got it twice. The first time (Delta) he almost got put on a vent. I’m B-something and never had it. Neither of us vaxxed. I have been taking the “protocol” vitamins. But so had my wife, and she got it.