As people interested in cooking we should all be aware of food hygiene and take it very seriously. As such I would recommend against your sister's methods, they are quite dangerous.
While bacteria can't grow or reproduce at 60C many types are more than capable of surviving; their cellular processes will shut down placing them in a kind of stasis or hibernation. So in a restaurant freshly cooked food will stay bacteria free when kept at 60C, however that is not what your sister is doing.
The problem with these methods isn't the period when the food is hot, it's the period after its been heated as in practice it will stay warm for some time and this provides optimum conditions for bacterial growth.
Let's look at another piece of evidence, sterilisation. If one wanted to sterilise a babies bottle say it would not be enough to just place it in boiling water, instead it is boiled for some time. In fact a safer way is to use a machine which steams the bottle (steam can be far hotter). The only way to be sure something is totally sterilised is to autoclave it, this involves both steam and high pressure and the items are kept in these extreme conditions for some time. Ofcourse autoclaves are only used when one requires there to be 0 bacteria, say in a lab or hospital. The lesson: bacteria are hardy things which are particularly hard to kill.
Final remark, better safe than sorry, particularly when young children are involved.