I have a samsung gusto, when I receive texts from my sister on her apple phone, half the time they are just a bunch of characters. I have no problem when my daughter sends me texts from her apple phone.
Is your sister using a lot of emojis or special characters? Is she using a special font or color, or otherwise formatting her texts? If the basic phone can't "translate" they will come through as random characters or square boxes, not the intended icon or formatting.
