Congrats!! How exciting for you :)
You don't need to worry about the baby-to-be and the nursing. What might happen, though, is that your milk may dry up and you'll need to supplement your 6-month old. I'm not exactly sure how that works; I suppose you need to just carefully watch diaper output and only supplement if that shows baby isn't getting enough.
I hope some btdt's chime in. Follow up with addl questions. Prayers for a healthy and happy pg~