It should be impossible, but someone else can access your WhatsApp mail. It would be best to learn how to know if someone is reading your WhatsApp messages and, in the next second, fix the problem. 
It's the most popular messaging app in the world. WhatsApp has 1.2 billion users and continues to grow at an astonishing rate. In addition to messages, video calls, voice calls, and picture or video transfer, WhatsApp has a high level of security. Before many other services included this feature, WhatsApp officials were proud of end-to-end encryption. This combination of technical terms is the most important assurance that only the sender and receiver of a message can know its contents.
WhatsApp encryption is so well developed that authorities in several countries have put pressure on the creators of the platform to find out the content of some messages, and they have refused. The situation is so delicate regarding the level of security built into WhatsApp that the online messaging service owned by Facebook had at one point been partially or completely banned in 12 countries. Voice and video calling via WhatsApp is banned in some countries. In other countries, the application does not work fully.
How is it possible for someone to read your WhatsApp messages?
Although it may seem incredible at first glance, it is possible for someone to receive your WhatsApp messages on their phone. The weird part of this reality is that WhatsApp has made a significant effort to not be able to activate the app using the same phone number on two smartphones.
The moment you change your phone and install WhatsApp using the same number, the app automatically shuts down on your old gadget and you can't even log in to read your old messages. The only solution is to reactivate it using another number. The same principle applies if you want to port your in-app conversations to another number on the same device. Apparently, the app developers have thought of everything. 
The problems started when the WhatsApp Web service was introduced, followed shortly by the WhatsApp application for PC and Mac. They act as a mirror of the whole set of mobile conversations. Instantly, the moment you pair your phone with your PC, it almost shocks you to see all your conversations immediately on the desktop. In other words, if you have a PC or Mac that only you use, the function is very good. Instead of reading your messages on your phone, change the Chrome tab in the foreground and see the confirmation of your appointment tonight, without taking your phone out of your pocket. Equally useful is the desktop client for WhatsApp that I have been using since its inception. For installation and configuration you can find a tutorial here - How to see WhatsApp messages on PC or Mac.
How do you know if someone is reading your WhatsApp messages and fixing the situation
As I said in the introduction, unfortunately, it is relatively easy for someone to receive your messages on WhatsApp if they had access to your phone for a few minutes. The secret lies in the function detailed above, WhatsApp Web. That option gave rise to a whole family of iPhone and Android applications that simulate the behavior of the WhatsApp application for desktop or web. Most of them are promoted as useful if you want to have two WhatsApp accounts on one phone. Some developers are a little more honest and write in the description that they can be used to spy on your husband, wife or child.
To make it clear that this is not just a myth, I will draw attention to a few examples that facilitate the theft of a WhatsApp account, before explaining how you can tell if you are a victim of such behavior. The first is for Android and is titled - WhatsWeb For WhatsApp Web, available on Google Play at this address. IPhone users can use Messenger for WhatsApp Web or WebChat - Chat for WhatsApp Web. They all do an equally good job and help you mirror your WhatsApp account from one phone to a second device with a completely different phone number.
The good news is that it is not at all difficult to tell if someone is receiving the same messages that you are receiving. If you have decided that you want to learn how to know if someone is reading your WhatsApp messages, the first thing you need to do is enter the application of the same name. Given that all the solutions detailed above take advantage of the WhatsApp Web protocol, in the Settings, in the lower right, you have to tap on the WhatsApp Web / Desktop section. Here you will see all the systems and browsers connected to your WhatsApp account. By default, they all have access to your entire conversation history.
If the list is empty and you are invited to pair a browser or desktop client in the WhatsApp Web / Desktop section, good luck. No one reads the messages you receive, unless they have physical access to your unlocked phone for a few minutes, in which case it's only your fault that you left it unattended. If you see names you don't know in that list, you have a problem and you can assume that someone close to you has read all your messages.
Although you can't do anything about what has happened so far, you can be sure that he won't read the ones you receive from now on. To do this, with a swipe gesture from right to left, delete the browsers and computers listed that are foreign to you. Eventually, press the bottom button marked Disconnect all sessions to clear them all in one go. I insist, although in the list you may see a system or browser like Chrome or Firefox, in reality it may be one of the applications mentioned above. Ideally, you should make a point of checking this list weekly. This is the only way to be sure that someone is not reading your WhatsApp messages.