If we don't know much about a person's history, we tend to assume they're deceased. Also, I'm sick of the dopplegangers.

I don't think so. Maybe Elijah or Klaus turned her into a vampire. Nobody can know.

Until that is verified, we leave her deceased. Since most points show she was sacrificed to bind Niklaus' werewolf side.

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...

stefan is the only one left because his line ended when he was turned 

Technically, if both of Silas and Amaras doppelgangers were born around the same time, each time they occured, Yes there should have been a Silas doppelganger around the same time as Katherine, before she became a vampire. But stefan was around in the mid 1800's so shouldn't there have been another doppelganger from Amara then? And if you see in Season 5, Tom is technically Elenas destined match but because Stefan became a Vampire he was the one who found her then. The Salvatore brothers have had their fair share with Doppelgangers, what with both of them having Katherine and Elena at one point.

Also if the dopplegangers appear every 500 years, shouldn't there have been a doppleganger between Amara and Tatia?  

Not true anymore as in Season 5 his doppleganger is Tom. Their cousins or any unknown siblings must have carried on the line. 

Tatia's blood was used to create 'The Original' vampires so she must have been killed by one of them are atleast had a fair wack of her blood taken from her to use in the wine they drank. 

Amara and Silas are the first of the doppelgangers, and because of the immortality spell nature found a way of creating shadow selves, also that they are 'soulmates' and would always find each other. Amara could have had a child before she fell in love with Silas or she could have had siblings who had children to carry on the family line to lead to the doppelgangers. Qetsiyah didn't kill Amara, she just made her the anchor to the otherside. 

Tessa didn't know the true doppelganger story. She is not the one who created the spell. Markos created and explained it. Doppelgangers were destined to find each other, NOT DESTINED TO FALL IN LOVE. Markos needed the blood of both male and female doppelgangers to undo a curse. He did not care to nor try to make them fall in love until season 5 when he found out love visions might cause Stefan and Elena to fall for each other again and bring them in the same area for kidnapping. Them falling in love failed, but he captured them anyway. Doppelgangers aren't meant to fall in love. They were meant to be human copies and sacrifices so Amara and Silas could live forever. That's all.