How can I communicate to my in-laws that the way they treat my wife is making her depressed?
You don't. They already know that they are hurting their daughter. But they do it anyway. First of all, you have to face this fact.
Of course, they don't think and make a plan like "Let's make our daughter feel worthless by leaving her out of our get togethers." But they instinctively synchronize and instinctively choose the actions that lead exactly to this result.
They do it because they are selfish. That is a very easy thing to do and everybody does it in some periods of their life.
Now, what do you do? You forgive. You forgive your in-laws for treating your daughter like she is some piece of furniture. You forgive your in-laws for not thinking about the results of their actions. You forgive that your in-laws do not love your wife.
Now, what do you actually say to your in-laws? What you say is something like this:
Remember that time when your daughter suggested a trip to that place? And then you all did it but left her out? I forgive you for doing it.
This will solve all your problems. You don't need to actually feel forgiveness to be able to say those words. Just say the words and then you will feel the forgiveness.
Or you can imagine yourself saying the words first. This way you can practice until you can actually say them. Maybe after each imagination you will also feel more and more forgiveness towards your in-laws.
This will bring forward true change. Discussions are something for the parliament. There is no place in your family for discussions. Forgiveness is the way.
Of course, after you have forgiven your in-laws and you see that they kind of are starting to change their behaviour towards your wife, then you can discuss something with them. Because now, you are friends. But as long as you are enemies, you must forgive.
My source is the account of Jesus Christ in the Bible. And this article about forgiveness in marriage. The article is about forgiving your spouse, but I think it is also applicable to the parents of your spouse.