Hi,
Thank you for your question. It sounds like you may be experiencing some of the more serious side effects associated with Vyvanse. Rapid or uneven heartbeats are cause for concern, and you should contact your doctor immedicately to report what you've been experiencing. You can read more about the side effects associated with Vyvanse here.
In addition, it is against federal law to share with someone else a medication that has been prescribed to you (or for someone else to share their medication with you). I would strongly urge you not to take your brother's medication, but to consult with your doctor and discuss the best way to treat your condition.
I hope this helps!
Best of luck,
Casey