My sister received an email from me with the contact being email@example.com. This concerned me. It seems that the email was sent to firstname.lastname@example.org also. I traced the email back to a carlos martinez. I do not know this person. I emailed him and no reply. If it was an accident he should reply. How did his email intercept my google account and my sisters. what is happening?
The situation seems to be that someone is pretending to be you, using a different email that looks like your main email. They are using this email account to email your sister. Are they pretending to be you in the emails? Are they asking your sister for money or sensitive information? Social security numbers, credit card numbers, etc?
I'll do my best to give you some general ideas of what I think might be happening. This definitely isn't a guaranteed truth, but there could be a couple of things if I understand correctly. These could also change likeliness depending on what the emails from the fake you to your sister said.
Could be someone pranking your sister. Maybe a family member or friend wanted to impersonate you and thought it would be funny to pretend to be you. I think this is unlikely, as that's not what usually happens on the internet, but it's possible.
Someone is trying to impersonate you to get money or information from your sister. This is called a "spear-phishing" scam. They know things about you, and your sister, so they are using that information to target you specifically. This information can usually be found fairly easily on social media, or in one of the many data breaches that have happened. Generally if it's from a data breach it won't be spear-phishing, unless you are a celebrity, a higher up in a company, or well off human being. I think this is a little more likely than number 1.
It could be a random phishing attempt which means they just got your sister's email and are sending out emails blindly. Because of potentially specific information, this could also be unlikely as generally these are unspecific and aimed at thousands of people instead of just one.
There are other things it could be, but it generally falls into one of the above categories. Hopefully this helps some.