I'd say there is a good chance you could be pregnant. Ovulation usually occurs in the middle of the cycle (approx 14-15 days into it from the date your period started) Sperm can live inside you for up to 4-5 days. (The girl ones tend to last the distance of time).

Not sure about the twins thing. I have a feeling twins normally skips a generation or two but you never know.

This is a great article on conception.
://www.bellybelly.com.au/conception/cervical-mucus-can-indicate-ovulation-fertile

Good luck either way. be sure to pop in and let us know how you go. There is some great advice on these forums