I haven't comed every inch of the thread, so I don't know if anyone mentioned or considered this.
Perhaps the name of D. is only given to males of the Gol D. Roger line. And speaking of lines, Robin mentions she is from a long line of Archaeologists. Couldn't it be possible for Robin to be Roger's granddaughter and know about it? Perhaps her mother was Roger's daughter, who fell in love with both archaeology and an archaeologist. Or perhaps Roger fell in love with an archaeologist and they had a kid. Being the king of pirates, I wouldn't be surprised if Roger sewed himself some wild oats. In that case, it is highly possible that Robin and Luffy could be cousins.
Or… perhaps Robin, Luffy and Ace all came from the same father. One who started a family in Robin's island home, then after some time, left to the East Blue to deal with something or someone, ended up on Luffy's home island somehow, and had a family there. Since it doesn't really mentuion anyone's parentage save for Nami, Usopp, and Chopper (kinda), it's hard to really tell.
Knowing Oda, anything is possible. The older sister thing seems the most feasible to me, though cousin seems to work just fine ^^