Most people think that they will marry just one person. They don’t. They marry many! I often tell my psychology students here at Santa Clara University as well as many of my single psychotherapy patients that before they say those two magic words (i.e., “I do”) they should carefully interview and spend a good deal of time with their fiancé’s parents, siblings, and other important family members. They should do so because once they get married it becomes pretty clear pretty fast that they are marrying a whole bunch of people who will now be very much a part of their lives. These folks will determine and be part of how they spend their holidays, vacations, weekends, and so forth for the rest of their lives! Think about it. Once you get married or partnered, all these extra people are now part of the deal and follow you everywhere (whether you like it or not).
You may have watched the 2002 comedy, My Big Fat Greek Wedding (see http://www.imdb.com/title/tt0259446/ for details). It was a fun film that depicted what it was like for a rather staid Caucasian man (an only child) from a British background to marry into a very large and full of life Greek immigrant family. It was a cute film for sure, but there is a great deal of truth depicted in the movie using humor to make some excellent points about marital relationships. Basically, when you marry someone, a whole lot of additional people (invited or not) are included in the package deal. Sure, they may seem charming and interesting at first, but see how you feel about them after a few years or decades.