I agree with your sister - but we are all different and our acceptance of risk is different.
Generally the rule is you shouldn't do anything that you weren't doing before you fell pregnant with regards to exercise.
But Im a kickboxer and we regularly get kicked in the stomach (behind a kick sheild of course but you still feel it). This concern me as when we start TTC I would be worried about injury but I dont want to stop if Im not pregnant Just sticking my two cents anyway cause Im no expert