Is the questioner obligated to return the phone's price to her brother after the death of her husband's mother, who used the phone, especially since her brother got her involved in previous matters?

You can resolve the first issue as you see fit within the bounds of Islamic law. As for the problem with your brother, limit your interactions with him to the realm of brotherhood and kinship, avoiding anything that causes you embarrassment or hardship. Do not be bothered by people's talk as long as you have cleared your conscience, acted in accordance with Islamic law, and avoided anything that compromises your integrity. Indeed, a believer is "a noble simpleton" (غر كريم), easily deceived due to their pure heart and noble character, while a wicked person is "a malicious scoundrel" (خب لئيم). Your greatest concern should be to please Allah, the Almighty, and to obey His commands and avoid His prohibitions.