"No." is a complete sentence. It gets easier to say the more I practice it.
My sister did plan my baby shower so I feel I owe her.
Not a level playing field. It's okay to pass on arranging the shower. It doesn't seem you were involved in this kind of drama and possible drug activity? Even without those added differences, you still get to say, "No".
Trust your gut instincts, mama. If you're feeling something is wrong and don't want your baby around this, it's perhaps a healthy move to take just as good care of yourself. When a child comes into our life, it's so very good to re-assert priorities in new ways.