"My sister did upgrade to 200GB but when I access my iCloud Settings on both iPad and MBP, I'm still at the default 5GB.":
Did you see the instructions in the "Share a storage plan with your existing family" section of Share an iCloud storage plan with your family - Apple Support
as suggested in my previous reply? See the excerpt below and have your sister follow the instructions:
On your iPhone, iPad, or iPod touch
- Go to Settings > [your name].
- Tap Family Sharing.
- Tap iCloud Storage.
- Follow the steps to share your existing plan, or if necessary, upgrade to a 200GB or 2TB plan.
- Use Messages to let any family members with an existing paid storageplan know that they can now switch to your shared storage plan.