It all depends. If the kids like the interaction with the father then I believe everyone would support him being around.
If the father is dangerous to the person with the kids or the own kids then things change.
Usually, if the kids have no fear then the father is not a threat to them.
Perhaps your sister should then find someone else to take the kids wherever.
If they produce statements on being scared, feeling threatened, then you can get interdiction order easily and for free in Australia and Brazil at least.
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