possible twins?

When I went for my first ultrasound at 4 weeks the ultrasound tech said that she seen 2 sacs & they were the same size. I came back a couple of weeks later & heard my baby's heart beat. I just went for my 13 week ultrasound yesterday & seen my single baby in my uterus. however my brother in law just commented on my Facebook announcement saying that he sees two babies, so here I am again gambling with the thought of actually carrying two babies 💕🤞 by the way the figure that we think may be the 2nd baby appears to be outside of my uterus but idk!