Is she sinful for telling her sister, "It is forbidden for you to sell it," intending to mean selling without a government license, given that she cannot remember whether she completed her sentence with "without a license"?

If by saying "haram" to your sister, you meant that it is not permissible for her to sell in school because the school prohibits it, then your statement is correct, and nothing is required of you. This issue falls under the category of restricting the permissible.

It should be noted that the phrase "haram" is often used by people, intending to mean "you ought not to" rather than a religious ruling. However, one should avoid being lenient in uttering it, as it is among the verbal prohibitions.

Sheikh Ibn Uthaymeen was asked about the ruling on saying: "It is haram for you to do such and such," and he replied that if the action is religiously forbidden, then describing it as haram is correct. However, if it is not forbidden, it is not permissible to describe it as prohibited, even verbally, because it might imply the prohibition of what Allah has made lawful, or it might imply an attempt to restrict Allah in His decree and destiny with the intention of a predetermined prohibition. One should avoid this word even if the intention is sound.