So why haven’t told us who’s sins Mary died for?
They don't believe Mary died - they believe she was assumed bodily into Heaven. Now we know that this type of thing happened at least twice in the Old Testament - Enoch and Elijah. But you would think that something that significant would have merited at least a passing mention by the authors of the New Testament, particularly by John, since he was the apostle to whom Jesus entrusted Mary. Out of the 5 books written by John, never was anything written about Mary to suggest she was taken directly into heaven without seeing death. Neither did John write anything about Mary being sinless or her being a mediator, etc. Kind of a major omission, don't you think?