My heart goes out to those who have married into extreme families and their dynamics. During the dating phase you may not have gotten a full picture of this from your fiance, but after marriage or permanent partnering, the dynamics showed up big time. Now you're flying through self-help books and canvassing blogs to find out what to do.
First, a background. Years ago a brilliant family researcher named Murray Bowen created a differentiation scale by which to assess families. Picture a line with 0% differentiation on one end and 100% on the other. Now let's define differentiation as the ability to be oneself and hold onto one's own identity and values, regardless of family pressures or sickness. Some families permit a high degree of difference, manifest loads of tolerance and flexibility, give a wide birth for individuality. They are closer to the 100% end. But those nearer the zero end are rigid and have many rules, (some spoken, others unspoken) manifest high anxiety when those rules are threatened, have insiders and outsiders, have secrets, and resort to extreme measures to attempt to control the thoughts, feelings, and behaviors of family members. Those extreme measures are usually some form of 1) enmeshed control and over-involvement or 2) it's opposite, threatened (or real) rejection, distance, or emotional cut-off. There may be siblings, cousins, step-kids, nieces, nephews all throughout the family tree that were forced to take either of these extremes--abandon individuality for someone else's sake, or leave the family entirely.
Therefore, these families usually manifest a high degree of ongoing drama and pain. They tend to react to most challenges, especially the expression of difference, in crisis mode. Drama with a capital D. And going into crisis is usually a group phenomenon. Think of a string of Christmas tree lights. When one goes out, they all go out.
It is usually sometime after a couple tries to form its own unique system that these extreme measures are brought into play. Families nearer the zero end of the continuum tend to have a hard time managing life transitions, like the coming of age and emancipation of a child, and the acceptance and integration of a child's partner. The question is, what do you do?
The answer is not simple or easy to capture in a brief piece of writing. But I would first begin by urging that if you recognize you've entered one of these extreme families, you sit down with your partner with a professional present and discuss what you're noticing and feeling. You need an agreement of a way of dealing with his or her family that does not involve extreme measures. Go gently but firmly because the children that come from these families can be quite defensive and conflicted about their own parents' behaviors. They may be fully aware of the effect of their family, but are deeply ashamed, unsure what to do, or afraid to risk any innovation. Or they may be in denial or completely in the dark. You may have to arrive at a set of ground rules to deal with family events, holidays, or day to day life. But you have to start with some acknowledgment from your partner of the scope of the problem. That's where a professional can help you.