I think if you mention that fraternal twins run in your family the doc should send you for a scan asap. My DH's cousin didn't go to the doc til about 16 weeks with her first. When he found out she was a fraternal twin he sent her for an emergency u/s. Only one.
When she fell pg the second time she had her first scan at 7 weeks - just to be safe.....She now has 7 month old identical twin girls!

Another friend waited with her 2nd pg til ahe was around 5 months. She never suspected a thing, but my MIL did. Sent for an emergency u/s coz her measurements said she only had 4 weeks to go......wrong! Still 4 months, but there were 2 babies hiding in there.
Her brother was a fraternal twin, but they lost one early in the pg & her aunt had fraternal twins, so no skipping generations there.

Go ask for a scan or a BT, really high HCG levels can be an indicater too.