pluto,Just to be clear, the 7th house is about the interaction of 2 people only, not 3 or more people. This interaction between 2 people is the relationship between these two people. If this doesn't make any sense, think of it this way. The lower part of the chart (houses 1 - 6) are about our "inner world". When we move into the 7th house we are, for the first time moving past the 6th house, past our inner world,a nd are beginning to become aware of the outer world. This awareness starts with our becoming ware of our individual interactions with each person in the outer world. So the 7th house is about those one on one interactions when we start meeting each person "out there".
In looking at relationship issues, it helps to look at the entire chart, rather than focusing everything on just one planet. But, if you have to choose one...for communication issues, look at Mercury (thinking, also communication/daily work) in each chart and in the synastry. The Sun is about how we express ourselves, but our Mercury has to do with how we speak...and how we are understood.
Our Moon (home, also emotions) focused in the 7th house (relationships) of another person indicates the other person feels we are emotionally "related" to them somehow. It can be a very deep, intimate connection, particularly if our Moon conjuncts (energy is combined with) their Descendant (others).
Mercurially emotionally relating,
Tim