1.Try VLC player(
VLC media player - Open Source Multimedia Framework and Player)
2.If it not work,you need to check the original video,maybe you can ask your sister to send it again.And what is the size of the video?Maybe it is too big to as a
attachment.
3.If your sister is not far away from you,she can transfer all stuff in her iPhone to your computer with SyncPod.Then she can share everything with you,of course you can also put it in your samsung cell phone and enjoy it.(PS:make sure the format is MP4,3GP or other format your cell phone support).