I know you're on to the next cycle now, but I could see the line.
However, it looked too close to the control line.
In regards to twins, the other lady was right about twins only running on Mum's side.
However, identical twins are NOT genetic - they're a random fluke.
Also, if you were 6DPO, a positive is pretty much impossible. The egg has to travel down the Fallopian tube, dividing and forming before it can implant. It has to become a blastocyst before it can implant, and hatch. This takes a minimum of 6 days, but I THINK it can happen 5DPO.
The embie then produces hCG, which takes about 2 days to reach your urine.
Here is a picture to illustrate my point - https://i1246.photobucket.com/albums/gg603/Wantingabubba/fallopian.jpg
HTH!