Is it considered a sinful supplication to pray that a sister's husband will not be able to marry other women, given that it negatively affects the sister's feelings? And is the husband's statement that this supplication fights the Sunnah of the Prophet (PBUH) correct?

No one is permitted to prevent someone who is allowed to have multiple wives from exercising his right to do so, whether by obstructing him or spoiling his proposal. Praying that he may not succeed in marrying another wife is impermissible, as it constitutes an transgression in supplication, which Allah dislikes. It is advisable to try to convince the brother-in-law to forgo marrying a second wife if he is not in need of it. If he is not convinced, then he should be left to his own way, and one should cease supplicating against him and seek his forgiveness if one has prayed against him previously.