family matters

I'm a big advocate for family. I'm a huge advocate for marriage. I think with a lot of relationships these days, people just have no interest in really putting in the work. They realize that the relationship isn't going to be easy, so they quit. I'm not a quitter. I don't give up on much in life. Here's the deal with relationships though: they're a two way street. Let's say Ryan and I have an argument. I can explain my side of things, I can talk, yell, cry, whatever, but if he's not willing to communicate with me and work it out, we can't. This isn't about Ryan though. Over the last 3 years, we have worked really hard to talk about things - even the things we don't want to. The things that cause conflict. The things we deeply disagree about. By talking about these things, we don't necessarily change each other's minds, but we do understand where each other is coming from. We find a way to connect through our disagreements and, where necessary, we compromise. So, you're married and you are working on having good communication with your spouse. Everything's going well and you have a strong, healthy marriage. Although this isn't easy to achieve, it's very possible because, hey, you picked each other. You have plenty in common, a strong attraction toward each other and a desire to be together - If you didn't you wouldn't have got married! Here's where things get messy. Insert family. Neither one of you picked your family. You may love your family with all your heart, but you did not pick them. You love your spouse and picked them, but you did not pick his/her family. So, how does that work?? Sometimes it's easy and flawless. Sometimes it's a giant mess. Whether family wants to admit it or not, for the most part, they will always side with their family members. everyone (within reason) has loyalty to their brothers / sisters, sons/ daughters, etc. So, let's say a woman has taken issue with her brother / sister-in-law, who's fault will this issue be? It might initially be the fault of the couple as a whole - but soon, it will become the fault of the in-law. Another example is a woman who has a close relationship with her mother or sister, who has taken issue with her sister-in-law, she will explain this issue, in detail, from her point of view to her mother / sister. Who's side, then, will the other family members take?
Where is the balance between having boundaries, "leaving and cleaving" and maintaining a close and strong relationship with your family? When does an unhealthy "in-law" relationship become toxic to the point that it should be severed?