I need a good excuse to convince lil sis not to come with to spend the weekend at my big sis's?

I understand that you cherish your time with your older sister and view it as an escape, but keeping your younger sister away from her for the reasons you have given is a great shame. Try taking her along and you may be surprised at the outcome. If you are absolutely unwilling to share your time with your little sister, maybe you can take her to Tania's so they can get to know each other while you go do something on your own.