Yeah but, the whole thing about Katherine's child and Tatia's child they could have been born with the Silas doppleganger...and then that child had multiple children and since you can have like four generations or more (especially in the 'olden days' because that's when people had children really young) it's possible that each time they are actually distant cousins...the other thing is that since dopplegangers are super natural, they go to the other side, so each time (lets just take Tatia for example) they meet the corresponding doppleganger on the other side and fall in love there. It would mean Amara and Silas both had children with other people, but it is possible that each time, the dopplegangers sometimes meet on the other side. This could mean, and I don't know for absolute surity here because although Katherine does seem to be dying, it is known that there's going to be a big twist at the end, but frankly I think she will die, it could mean that Katherine (as long as her child wasn't with Silas doppleganger), will find the 'correct' corresponding doppleganger (because it is clear she loves Stefan and all, because they are dopplegangers meaning they were going to be attracted somehow, but they are not true lovers, since he is Elena's) on the other side...