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 sister luckily had a light case because she had Guillain-Barre in the past and cannot tolerate very many meds.