First off... sorry to hear about your Daughter.
Regarding your Grand Daughter, the greatest underlying cause of High BP is being overweight.
One of the best ways to gage if she is overweight is to calculate her Body Mass Index (BMI). What is her weight and height...? These two numbers can be used to calulate her BMI.
It's important to talk about weight when it comes to blood pressure. I know that a person's weight does not make who the person is... especially a for a child... they are all beautiful.
I know a child's beauty is not based on weight... but BP generally is.
And a child being on the swim team does not mean that the child is not overweight. All my children have been active swimmers for 8 years... two swim on both the High School Swim Team and on the High School Water Polo Team, and all of my children swim on a recreational swim team. They are basically swimming year round.
Given my exposure to swimming, I'm very familiar with the physiques of the kids who swim... and there are quite a few good swimmers that are clearly overweight... maybe even obese... both boys and girls. So I it's important to be factual regarding weight.
I'd like to hear how high the BP really is Do you have her blood pressure numbers...? Maybe her BP is only borderline high.
At age 32 when I was first diagnosed, my BP was 160/110. I bet it was high when I was 20 given how extremely high is was at age 32.
Anyway, if you would like to share any of those numbers, folks may be able to give a better opinion.
Best of luck.