Claudius said this referring to Gertrude, because he married his brother’s wife.
This quote makes light of the fact that Gertrude is once was Claudius’s “sister,” or at least his sister in law, and is now his wife. Claudius comments on how she married while still grieving for her husband.
Have we, as 'twere with a defeated joy,--
With an auspicious and a dropping eye,
With mirth in funeral and with dirge in marriage,
In equal scale weighing delight and dole,--
Taken to wife (Act 1, Scene 2)
Claudius making light of King Hamlet’s death, and Gertrude’s marriage, is one of the reasons that the prince cannot stand him. Prince Hamlet is irritated with his new stepfather all right. His father’s ghost has accused him of murder as well. Gertrude may know nothing of this, but she goes along with Claudius anyway.
Claudius may not have trouble with Gertrude, but he has plenty of trouble with Hamlet. He thinks that Hamlet is just plain mad (the crazy kind, not just the angry kind, although Hamlet is angry too). He does not approve of him moping around for his father, and tries to talk him out of it by reminding him that he is a prince.
We pray you throw to earth
This unprevailing woe, and think of us(110)
As of a father; for let the world take note
You are the most immediate to our throne … (Act 1, Scene 2)
If I were you, Claudius, I would not remind Hamlet that he can become king by killing you! Hamlet’s eccentricities only get larger as the play goes on, and Claudius tries to convince himself and others that Hamlet is just lovesick. Then he tries to bump Hamlet off. Hamlet is too wily for that though, and kills the messengers instead. Unfortunately, Hamlet’s plans to avenge his father’s death are so overly intricate that most of the characters in the play die too (including Hamlet) by the time he finally accomplishes his goal.
This quote "Our sometime sister, now our queen, ... have we taken to wife?" is said by Claudius and he is playing with his words trying to make it seem like the situation was completely out of his hands and did was best. In this paragraph Claudius talks about how he only married his "sometime sister" (which refers to the queens period to were she was once his deceased brothers wife)because it was "advised" that he do so since she was a "jointress to" the "warlike state."