How can we be happy in heaven knowing that some of our loved ones did not make it there?

The happiness of heaven cannot be equated with earthly categories and prerequisites. Exactly how we will be happy in heaven cannot be explained to us here. Scripture describes heavenly happiness as: What no eye has seen, what no ear has heard, and what no human mind has conceived" -- these things God has prepared for those who love him (1 Cor 2:9). 
 
Jesus also cautions the Sadducees, who tried to project the earthly realities of marriage and family into heaven. He said You are in error because you do not know the Scriptures or the power of God (Matt 22:29). In other words, and for our purposes here, we have to admit that our grasp of what Heaven is, and how it will be experienced, cannot be reduced to, or explained merely in terms of how we are happy now. 
 
That said, some have speculated (and it IS just speculation) that the happiness of heaven, even despite missing family members, will be possible in light of the deeper appreciation of God’s justice that we will have there. Surely we will concur in heaven with all God’s judgments and in no way incur sorrow on account of them. 
 
Hence, we will see that those excluded from heaven are excluded rightly and have really chosen to dwell apart, preferring darkness to light (Jn 3:19). And while it may currently be mysterious to how this will not cause us sadness, God does in fact teach us that he will wipe every tear from our eyes (cf Rev. 21:4).