Conflict with wife due to bad behavior of her maternal relatives, bad relationship with elder son

Your frustration and helplessness seems to be evident in the presence of a marriage that's has gone bad. That along with your child turning against you is quite a painful situation.

It looks like your wife is letting the external influences of other family members affect your marriage a little more than it should. As much as the extended family matters, a marriage and its contents are primarily between the two partners.

In my opinion, talking to your wife, in the presence of your son, if you think is okay, can help define the actual problems. Many times one sour incident can be the curtain over various other issues that have not been communicated well or have been neglected by you. It is possible that you are contributing to the situation, unknowingly. A clear conversation can help define clearly, where the issues have risen from and what is continuing to fuel that fire.

You can take a rationale call after that. If it seems like your marriage requires effort and communication, you and your wife can mutually decide to dedicate that too each other to rework the foundation of your marriage.
I would also recommend marriage counselling/ couples therapy. Often therapy creates a secure space for both individuals to be heard and understood. It will also serve as a channel for the two of you through the long process of rebuilding, should you choose to do it.

Through the whole process, it is important for you to understand and accept that to rebuild your relationship, effort is required from both of you with as little interference as possible from extended family members. It requires effort over an extended period of time as well, so you both have to exercise patience and caution through it, but form a common ground to the same end goal. As long as you both have the same end goal, the process should work over a period of time.

Thank you for reaching out for help and seeking alternative opinions and advice. I hope your situation gets better and you're able to find a happy ground in life.