I’m thankful your marriage has been happy and fulfilling; God gave marriage to us, and this is how He intended it to be. It’s no accident that the Bible compares Christ’s deep love for us with the love that should exist between a husband and wife. (See Ephesians 5:25-33.)
But if it’s God’s will for your sister to remain single (as it is for some), don’t worry about her happiness. You can be confident that God will provide other ways for her life to be fulfilled and happy. Yes, you experience happiness through being with your family and doing things with them—but she may have opportunities to do things and meet people you’ll never be able to, because you simply don’t have the time.
Remember: Jesus was not married, nor was the Apostle Paul. Paul, in fact, urged people not to enter marriage thoughtlessly or without being certain it was God’s will. Marriage was indeed God’s will for most—but for others (he said) singleness was best, so they could “live in a right way in undivided devotion to the Lord” (1 Corinthians 7:35).
Urge your sister to seek God’s will for her future; God loves her, and He knows what is best for her. Urge her too to consider becoming involved in a church that has an active singles ministry. In addition, respect her decision, and don’t irritate her by constantly bringing it up. Pray for her also (as you should do for all your family), and go out of your way to include her in family gatherings.