Twins without a family history?

At the moment my husband and I are not actively trying to conceive. I will be finished with my degree next year and then we are going revisit the baby talk.
About 10 years ago when I was 18 I went to a psychic on the boardwalk with my high school best friend (I know). The woman stared at me for a few then said "you will have twins....boy girl twins and you will marry your high school sweetheart." At the time I didn't have a boyfriend, so I didn't read too much into it. I was more freaked out than anything. 8 years later I married my highschool boyfriend. 
My family has experienced many tragedies within the the past few years so we have been connecting with a well known psychic medium in our neighborhood to help us heal spiritually. In our family reading today she mentioned that new life will continue to come into our family a year or so after the birth of my sisters little girl this October. Everyone in our family is a girl.... and every girl has only had girls (so far). I asked the psychic medium if she could see a specific gender, and she said "I see twins, not identical - I see boy/girl twins." It may not be related directly to myself and my immediate family but I am curious of situations in which twins have occurred without any knowledge of twins running on either side of the family. My husband and I cannot think of twins on either sides of our families. I guess twins have to start somewhere. Boy/Girl twins would be awesome. 
Does anyone else have boy/girl (or b/b, g/g) twins without any knowledge of twins running in their family? I'm curious to see the chances of this happening to myself of one of my sisters. 
Thanks for reading!