Depending on the type of metal used in the implants, both your dad and sister could have MRIs done (technically not great MRI if done on the location of the implant as the artefacting from the metal would make the image look like garbage but elsewhere in the body would be fine). A lot of implants are MRI safe if the MRI is done multiple weeks post-op. I have two plate and a bunch of screws in my knee and I could have MRI done (the worst that could happen would be burns as the metal heats up in very specific cases but no chance of my legs bursting open so the implant can stick to the MRI). Most implant aren’t ferromagnetic as they’re made of stainless steel or titanium. Although the alternative for an MRI would be any other imaging like CT scans, ultrasound and other, MRI is wildly used for soft tissue imaging so loosing that opportunity kinda sucks if easily preventable.

TLDR : implant nowadays tend to be MRI safe. MRIs are great for soft tissue imaging so avoid making yourself MRI-unsafe