Is my sister's purchase of gold with zakat money I gave her considered to invalidate the distribution of zakat, and is it permissible for me to give her zakat in the future given that she is married and her husband is employed?

You are correct in paying Zakat (definition) to the widow and orphans if they are poor. As for your sister, it is permissible to pay Zakat to her if she is poor and her husband's income is insufficient for her, and she may use it as she wishes. However, if she is not poor, it is not permissible to pay Zakat to her. If you paid it to her out of ignorance of her financial situation, then it is safer to re-issue the amount and pay it to those who are deserving.