There are few joys in fatherhood that compare to hearing my child say "I love you, Daddy," but one that comes close is seeing my children loving each other. My four-year-old daughter and two-year-old daughter have reached an age that they love playing together. They will run around the house squealing and laughing and smiling and experiencing genuine joy being together. The baby has recently grown enough for her personality to begin to emerge. She smiles and giggles as I play peak-a-boo or shake a rattle for her. The two-year-old, who never misses an opportunity to be a 'big helper', loves to imitate me by playing these games with her sister as well.
When I describe these developments to friends and family, they will often comment on the convenience that comes with the children entertaining themselves so they don't require constant interaction. They are right that this is convenient, but there is a deeper joy that comes from watching my children play. Seeing my daughters find joy in their love for one another brings me genuine joy as well. I feel a deeper love for my family when I see them loving each other.
We have been watching Bishop Barron's The Pivotal Players with a church group and we recently watched the video on Saint Catherine of Siena. When she was considering leaving a contemplative life to serve those in need around her, she was concerned that she would grow more distant from God. Then she heard Jesus say:
I have no intention whatever of parting you from myself, but rather of making sure to bind you to me all the closer by the bond of your love for your neighbor. Remember that I have laid down two commandments of love: love of me and love of your neighbor... On two feet you must walk my way, on two wings you must fly to heaven. 
My experience of growing in love for my family as my daughters grow in love for their sisters in my domestic church gives me a deeper appreciation for the profundity of this idea. Just as my bond with my daughters is strengthened by their bond with their sisters, my bond with God is strengthened by my bond with my neighbor. The two go hand in hand. I understand how Jesus could say that what you do to the least of these you do to him. If you do something to make my children happy then you make me happy as well. If you mess with my kids you better be ready to mess with me too!
This experience has held as I've grown older with my own brothers and sister as well. I love my parents and I love spending time with them. I also love spending time with my siblings and I feel closest to all of my family when we are all together. Even when my parents are not physically present with us, they are present in a way through my siblings since we all carry parts of our parents with us. Since we each possess different parts of my parents, my parents' presence with us is greatest when we are all together. My love for my family is also multiplied when I see my brothers and sister playing with their nieces, my daughters. The exponential effect of all this love draws in everyone in our domestic church and the family love shows me how we can grow in our love for God through love of our neighbors.
As my daughters learn to interact with each other, it also prepares them to serve their neighbors. The fraternity between siblings can lead us to a deeper fraternity with our brothers and sisters in Christ as Pope Francis explained:
In the family, among siblings, human coexistence is learned, how one must live in society. Perhaps we are not always aware of it, but the family itself introduces fraternity into the world! Beginning with this first experience of fraternity, nourished by affection and education at home, the style of fraternity radiates like a promise upon the whole of society and on its relations among peoples. 
As they learn to share, to forgive, to encourage, to play, to converse, and to dream together, they learn skills they will one day be able to use with other students, coworkers, friends, and neighbors. When they love their neighbors they will deepen their bond with their heavenly Father, just as today they deepen their bond with me as they grow in their sisterly love.
 Raymond of Capua, The Life of Catherine of Siena, trans. C. Kearns (Wilmington, DE, 1980), 8, as quoted in Paul Murray, “St. Catherine of Siena: The Mystic,” in Catholicism: The Pivotal Players Leader Guide, vol. 1 (Word on Fire, 2016), 6.
 Francis, “Brother, Sister: Words Beloved to Christianity,” in Pope Francis on the Family: Weekly Catechesis December 2014 - September 2015 (Manassas, VA: Trinity Communications, 2015), 24, accessed December 23, 2016, http://www.catholicculture.org/ebooks/Pope-Francis-on-the-Family.pdf.