Argthtjtr

First off, I read the book around 4 years ago and I'm pretty sure it came out not long before I read it, so I'm thinking the release date is somewhere around 2012. And I believe there are at least two books in the series.
And it's a post-apocalyptic book.

The first book starts by revolving around this one girl who is scavenging her surroundings for food. She describes a lot of stuff in a diary (I think) and how the world came to be. I think that aliens came to the Earth who look like human beings and a lot of the suspense drawn from the book is if you can trust people you meet or not because they could be aliens. Towards the beginning of the book, she's looting a gas station but finds some guy who killed himself (or is about to) and on his neck is a silver cross necklace. I don't know why but this is important and she either takes it or looks at it for a while and remembers something about her past. There are a few flashbacks to her past and in it, she reveals she has a brother and she can't find him. Most of the first book is based on finding her little brother.

Okay, I'm gonna switch gears a bit here. So in this world, there are a bunch of camps of people trying to survive. I believe the brother of the girl lives here with a boyfriend that she had in high school maybe. Or a guy that likes her. I'm pretty sure he likes her, but she doesn't feel the same way maybe. There are also these government camps that say that they take in families (or just kids idk) to keep them safe. What they actually do is take the kids in on buses, sort them, and then put chips in their heads to track down humans. I'm pretty sure this "government camp" is actually the aliens.

Well, so the brother (his name might be Luke or Sam. I'm just going to call him Luke from now on) is shipped off to this camp with the guy that knows Luke's sister (maybe previous boyfriend). They are put into buses and taken to this weird facility and told to sit down on the ground with hundreds of other kids. They are given numbers and then sorted into two groups. Those fit enough to be able to track down humans and kill them, or those who are not. I don't remember what happens to those who are not.

Okay switching gears back to the girl now.
So she's looking for her brother and I'm pretty sure she finds the camp where he is and they meet, but the government people come but she's suspicious of them and hides. So the government people are taking away some of the kids and Luke goes but she tries to stop him but they somehow get him on the bus and are then hunting down the girl. She shoots one or something and dives into bushes and they give up trying to find her and she overhears them talking about what they're going to do and something they say reveals that they're maybe aliens or planning to do something bad. I don't know. My timeline might be completely off so bear with me.

Somehow in the near future, she is terribly injured, but this guy takes her into his cabin and nurtures her back to health. He goes out every night with a rifle to hunt for them, but she's really suspicious that he's an alien. I think at one point she tries to kill him. He starts teaching her about how to use a gun (maybe) and she starts falling in love with him. They might sleep together or something but they are really close then. She insists on going to save her brother but he says she needs to rest and says he'll go get her brother. Long story short they both go and at some point, they find out that he really is an alien (maybe) but still fell in love with her and is now on her side.

They go to the government facility, save Luke and find her previous boyfriend and there's a whole love triangle or something like that. They're escaping in a Humvee and the girls in the back and the alien guy she's in love with is running after it because the facility is blowing up and stuff and they're trying to get away but also get the alien dude so it's pretty dramatic there. I think that's the end of the first book.

This might be in the second book, or it might be the first book, or it may be another book altogether lol but I think they're in the same universe. This is all I remember of the potential second book (I think I only got around halfway through it and never finished it): There's a rebel group (I think with all the previous people) and they're hiding from the aliens/people with chips. At some point, they put chips in their head so the people with chips think that they're on the "government's" side but then kill them.

I've never read the book, but I saw the movie adaptation of The Fifth Wave (2013) by Rick Yancey, and this matches everything you've described. It's the first in the trilogy of the same name.

From Goodreads:

After the 1st wave, only darkness remains. After the 2nd, only the lucky escape. And after the 3rd, only the unlucky survive. After the 4th wave, only one rule applies: trust no one.

Now, it's the dawn of the 5th wave, and on a lonely stretch of highway, Cassie runs from Them. The beings who only look human, who roam the countryside killing anyone they see. Who have scattered Earth's last survivors. To stay alone is to stay alive, Cassie believes, until she meets Evan Walker. Beguiling and mysterious, Evan Walker may be Cassie's only hope for rescuing her brother-or even saving herself. But Cassie must choose: between trust and despair, between defiance and surrender, between life and death. To give up or to get up.

...

Evan is an alien who was sent on Earth years ago to "prepare the invasion" or something, but his love for Cassie overruns this order and he helps him take her brother back. I think he died in the process.