Could i possibly be the father????? i wanna be?

My girlfriend and i are expecting our first child we have been together on and off for the past 3 years we got back together at the end of august in 2010 she had her period in august and a second period in september around that time we found out she was pregnant we also found out her older sister was pregnant thru this whole pregnancy ive stood by her side alot of people are puttin it in my head that the apple of my eye maY NOT BE MINE and it hurts but now the dates dont add up to her being mine what are the odds that the doctor got our due date mixed up with her sisters due date because her sisters due date adds up perfectly to her baby being mine (im not sayin i slept with her sister) and our due date adds up perfectley to her baby being her mans child (im not sayin him n my gurl slept together) im jus wonderin if the doctor couldve possibly got our due dates mixed upo due to both my girlfriend and her sister have the same last name and the same doctor????? and her sister is way bigger (baby bump) then her so is her sisters due date our due date and our due date is her sisters due date please help me figure this out cuz i cant go thru the pain finding out my daughter aint mine):