I admit you are in a very difficult and delicate situation. But, mind you, you are not alone in this situation. There may be many more like this in this world, because the situation is more or less common in many households. But most of them carry o and get out of it some how in due course. So with a firm will and sincere focus and slight adjustment in your life style and behaviour you also can overcome this bad situation and come out happily.
What is needed is 'mending' of slightly ruptured relationship. If mended on time, there will not be any major break and damage.
Just try to recall when the relationship between you and your wife started to sour. Then you can find the reason for that.
Your wife may be a feeling that you are neglecting her. For that she finds a reason that you are having other relations. So the priority is that you spend time with your wife and children more. Convince her with your actions
and truth that you are still loving her as before, and the reason for not spending more time with her is due to work occupation etc. For her sake take a holiday and go out with her and children. Both of you open up your minds and clear all the doubts and suspicions. She may be feeling burdened or bored at home. If she is overburdened with household work, try to help her or find some way to educe her burden by sharing some of them. If you need home appliances etc buy them and hep her.
Your elder child is now ten years. Being a girl, she needs mother's care and proximity most from now. You may not be able to replace that. So you may feel that the child also is siding her mother. Ultimately that will drive you to a feeling that nobody likes you. Prevent that. Spend time with your children also helping them in their studies, taking them to places,caring about their food and health etc.,
When you feel stressed you will think about divorce. Quit that thought. Divorce will not be as easy as you think, and even if it comes, it will come with a cost- financial, mental and social cost. So as far as possible try alternate and easier healthy solutions first. They will definitely work. Your case is not that worse. It is mostly due to lack of proper communication and interaction.
Start the first step of reconciliation from your side. You do't have to lose anything. But only to gain your life back. Tell your wife that both of you can forget whatever happened and lead a happy life by adjusting more.
Forget about your in-laws. Give them the respect they are deserving only. You take command of your life. If they try to meddle tell them to get out of your life.
If your wife is not amenable to reason and do not change her behaviour, tell her with sternness that you will better split and divorce.
But I do not think things will go that worse.
Forget your ego. Reconcile for the sake of the children and your own peaceful life. Never affect the children about the bitterness between you and wife. Slowly change yourselves. Your wife will also automatically respond.
(Note: when some females come to your home, do not spend more time with them ignoring your wife. Let them spend more time with your wife. Let your wife only bring them to talk to you in her presence. That will remove the suspicion.)