What should I do, as I invoked death upon my siblings and then regretted it? Is there an expiation for that, and how can I be sure that Allah will not answer my supplication?

Your prayer for the death of your siblings is an aggression in supplication, which is forbidden, and you incur sin thereby. You must repent to Allah Almighty, and this supplication will not be answered because it is an injustice. There is no expiation upon you except sincere repentance and not returning to such an act.