I would say that what the two families realize too late is that their feud is stupid. They do not manage to learn that until their kids have both ended up dead.
We do not actually hear them say that they have learned too late. But we do (in the last scene) see them make up with each other. We see them try to become friends because they know that what they were enemies over (if they even know) wasn't as valuable as the lives of their kids.
So that's how I would answer this -- they realize too late that they should stop hating each other.
In Romeo and Juliet, the Capulet and Montague families realize the heart-wrenching toll their feud has on each family and the town of Verona as well. The Capulet family has lost their daughter and cousin (Juliet and Tybalt) while the Montague family has lost their son and mother (Romeo and Lady Montague). In addition this feud has led to the death of Paris and of the Prince's cousin, Mercutio.
In the prologue, it states the feud between the Montague and Capulet family is ancient. Both side probably do not even remember the start of the feud. Only the death of their children was enough to overcome the hatred between the families.
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes(5)
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents’ strife.
The fearful passage of their death-mark'd love,
And the continuance of their parents’ rage,(10)
Which, but their children's end, naught could remove
In Act V, Scene III, the Prince rebukes both families by pointing out that the end of their feud has resulted in the needless death of their children. In addition, he also states that because of this feud, he himself, lost two family members. Everyone has lost because of this feud. Everyone has been punished.
Where be these enemies? Capulet, Montage,
See what a scourge is laid upon your hate,
That heaven finds means to kill your joys with love!
And I, for winking at you, discords too,(305)
Have lost a brace of kinsmen. All are punish'd.
After this is pointed out, the Capulets and Montegue's agree to end their feud and to honor the memory of their children.
O brother Montague, give me thy hand.
This is my daughter's jointure, for no more
Can I demand.
But I can give thee more;(310)
For I will raise her statue in pure gold,
That whiles Verona by that name is known,
There shall no figure at such rate be set
As that of true and faithful Juliet.
As rich shall Romeo's by his lady's lie—(315)
Poor sacrifices of our enmity!
1) this question is not to clea but if you are talking about the fued then here is the answer:
This was a family rivalry that had been ongoing for years before the story of "Romeo & Juliet" begins. The cause behind it was never explained by Shakespeare. Romeo was a Montague; Juliet was a Capulet.
Their children fall in love but lose their lives as a result of the feud.
Shakespeare, however, didn't actually invent this story, someone else did. The cause may have been explained by the unknown author, so I will update if I find out more.
2) Romeo's Dagger. in the Vault, after finding Romeo Dead beside her.
3) The Friar explains everything that has happened. Montague and Capulet finally see how much damage their feuding has done so they decide to end the war between their houses. As the Prince says, "It is a glooming peace this morning with it brings. / …For never was a story of more woe / Than this of Juliet and her Romeo. (V, iii, 305-310)