Is a girl sinful if she is unable to make up for missed days of Ramadan due to intense heat and difficulty of fasting, knowing that she sometimes delayed making them up out of negligence, and she intends to make them up in the following Shawwal along with feeding a poor person for each day?

The missed days must be made up after Ramadan. If the delay is without a valid Shar'i (Islamic legal) excuse until the next Ramadan begins, then the negligent person must make up the days and feed one poor person for each day, along with repenting. This is the view of the majority of scholars, in contrast to the Hanafis, who permitted the delay absolutely without sin. The evidence for the majority's view is the statement of Aisha (may Allah be pleased with her) and her emphasis on making up the fasts in Sha'ban, as well as by analogy to delaying the obligatory prayers. Furthermore, it is based on what was narrated from the Prophet (peace and blessings be upon him) and the Companions regarding the obligation of both making up the fast and paying the fidyah (expiation) for the one who was negligent.