Twins....how late did you find out?

👧🏽Lindsay👶🏽 • 👰🏻🤵🏿💍 2012. 👧🏽🎀 1/2016. 👼🏽MC Oct ‘17. 👶🏽🎀 #2 born 12/2018! #2 was born with medical and special needs. My girls are the joy of my life💕
I am 17+4 weeks and my sister is convinced I am having twins. I have already had two ultrasounds where I only saw one. But I have been feeling movements for a long time and in two very opposite spots (one down low and one higher on my left-middle). 
She has two children a boy and a girl, which I think is why I have been having dreams about having twins or two kids.....but she's convinced that it's a sign lol. 
Has anyone ever found out really late that they were having twins? If so....how far along were you??