During the Renaissance, the setting of "Romeo and Juliet," marriages were arranged by the fathers of the young couples. In Act I, Paris, a kinsman of the Prince, has spoken to Lord Capulet about marrying Juliet, but Capulet hesitates since
My child is yet a stranger in the world--/She hath not seen the change of fourteen years./Let two more sumers wither in their pride/Ere we may think her ripe to be a bride (I,ii,7-10)
That Capulet changes his mind about Paris's waiting later in the play may be evidence of the aristocrats' having such arranged marriages: They wish to assure that their children made the proper choice of person. That is, they desire that their children marry within their class. Also, financial considerations play a big part: dowries, properties, political alignments, etc., were all factors. Thus, Capulet may have changed his mind about Juliet's marrying so young as he realizes that she is enamored of his mortal enemy's son, Romeo.
Regarding nurses, often improverished women of noble families would be allowed to live with their affluent relatives and act as nurses to their nieces and nephews. The Nurse's being a relative is probable since she is even afforded a servant of her own Such is probably the case with the nurse in "Romeo and Juliet" since she speaks in such familiar ways to Juliet, advises her, and dotes on her as an overly fond relative. Also, because she is a relative, she is not dismissed from the household when Juliet is no longer in real need of her.