Doesn't she later go on to say that she wouldn't do it for a child or for a husband as she could always get another husband and through him more children??
but because both her parents are now dead, it is not possible for her to ever have another brother
which is why she sacrificed herself to honour him in death.
But even though she says that, most people believe that she is not in the right frame of mind
and is therefore not telling the complete truth.
This is evident by her passion and values of family throughout the entire play.
Hope that helps!!
That is at least what we learned in my class.