Synastry is the subject that deals with your relationship with the other. The other can be anyone your lover, husband, sister, brother, mother or father. Planets change their roles as per the relationship. But one thing that you need to keep in mind while you read the house overlays is that the house person is the giver. It simply means that the planet opens the lock of that particular house's energy. The planet can be a welcome key or a pestering key but it will always be the key and never the lock. Planets in your fourth house open your deepest emotional core, so a mars there will inflict a frustrating wound. Pluto would inflict deeper psychological trauma. Venus your need to have a luxurious home with the other. Moon is most comfortable here, so you both can live "happily ever after" with deep emotional bonding if other aspects permit. Saturn will be the old father Williams who will restrict your movement and will bring stagnancy along with stability. Mercury will trigger the need to travel with the loved one, considering a non-permanent dwelling. Jupiter will trigger long-term travelling with frequent changes of home. Always, buying bigger and better homes while changing locations. Sun will trigger your need to have a royal, cultured home with your significant other as you get awed by his/her authoritarian behavior. Neptune will create a fantasy of home with the other. You will imagine having the other person inside your home while escaping from the world. Uranus will make you a gypsy that breaks away from the very convention of having a home. You will be introduced with a foreign culture that will be unusual, unorthodox and unpredictable.
Understand that each house carries it's own energy and keyword. Each planet carries it's energy and keyword too. Equate the two and understand the lock and key principle to know how you will be affected. I have listed the energies and keywords of the houses and planets.
Sun- Illumination, Success, Royalty, Pride 5th house- Fun, Creativity, Children, Sports, Fame, Ego
Sun in the 5th house in synastry will make you proud, successful, enlightened just like a royalty! Saturn will be the exact opposite.
Stelliums in synastry tend to trigger your house energies in an almost explosive manner. For e.g somebody's stellium in your first house will boost your ego and confidence to the tee. It will bring out your most competitive and aggressive side on the shore. You will feel like you are on the top of the world. In case of the other person's stellium being in your 5th house, your leonine traits will come on the shore. You will feel like a royalty. You will not hesitate in enjoying gambling, sports or theaters. You will plan to have kids with this person and will also love to bask in the glory all the time. In the 7th house, you will be more focused on forming partnership with this person whose beauty/looks have captured your attention. You will become more refined in your approach and would become more serious about marriage than you were before. You would want 100% commitment guarantee from this person.