We do them as we can as families grow. My parents are up to 8 grandchildren now, with my other sister's wedding next summer and a strong desire for more cousins expressed weekly around here.
And I loved spending time with my sister. Meals were easier with my sister in the kitchen. We planned and cooked and chatted. Phone calls are nice, but three days together is a great way to pick each others' brains on just about everything. So, while living next door would be crazy fun, vacations to each others' homes are the next best thing.
It is visits like these to relatives and cousins that I remember most fondly from childhood, and I am glad my children get to experience them as well.