Apparently the picture that has been circulating the internet the past few weeks does NOT show Quicksilver's sister, The Scarlet Witch, sitting on her brother's lap. At best, this is a homage. In an interview with GEEKS OF DOOM, Xmen director, Bryan Singer, put it plainly.
“There may be a reference, but no, no. There are 26 main characters, and with seven of them, there were a lot of restrictions in terms of schedules.”
He went on to elaborate that Quicksilver didn't even have that much screen time. So, the best we will probably get is a quick look (forgive the pun) of the Mutant Speedster! But nothing concerning his sister, Wanda.