<p>crouch88 handled the second sentence already.</p>
<p>For the first sentence, your thinking that the constructions should be parallel is correct. I think that a second “to” would be required if “swim” were replaced by a much longer construction, so that it became less clear that “sit” and “swim” were the two options. Since there’s nothing else intervening, it’s ok to omit the second “to.” Or so I surmise, if these came from the Blue Book.</p>