Is it considered a sin for someone who is late for Fajr prayer due to overwhelming drowsiness and their assumption that there is still time, similar to the sin of intentionally abandoning it, and is it worse than fornication, drinking alcohol, and murder?

The matter occurred inadvertently on your part, and you did not intentionally let the prayer time expire. Error is forgiven for this Ummah. Therefore, your ruling is not the same as one who intentionally leaves the prayer until its time expires. However, you must pay attention in the future to avoid repeating the mistake.