Blood is thicker than water, which is easy to see when it's spilled all over the floor at the end of the play. Hamlet dwells on issues of incest and revenge, both of which just might be taking the familial bond a little too far. You shouldn't have sex with your family—pretty much everyone can get behind that—but maybe, Hamlet says, you shouldn't be murdering for them, either. So, what's left? Normal families? Nah. This is Shakespearean tragedy; keep looking.
In Hamlet, parents can't be trusted to look out for their children, especially when matters of politics are involved.
Hamlet is sad that his dad's been murdered, but he's more sad about his mom's remarriage to Claudius.