Are leaking rubber seals more common in hatchbacks than sedans?

There’s absolutely no correlation between body styles (hatchback versus sedan) and the propensity for leaking brake cylinders. The difference between your mum’s and your previous car and the Tiida is probably all down to the quality of the parts, not whether it’s a hatch or sedan.

Brake cylinders leak when the rubber seals inside them fail. But because the cylinder is hidden within the brake drum, a leak is often only found when the car is being serviced or inspected for a roadworthy certificate. And as you’ve found, that’s often too late to prevent the brake shoes becoming contaminated by the leaking brake fluid.

The best way to avoid leaking brake cylinders is to have the braking system flushed regularly. It’s also important to replace any leaking wheel cylinder with a good quality unit. I’m tipping the reason your cylinders failed the second time was because the original ones were replaced with cheapies (possibly sourced online) which simply weren’t as good as the Nissan originals. Always beware when buying replacement parts of any sort that they’re good quality, particularly when it comes to critical safety systems like brakes.