It depends on your DD, but do talk to her about it, she certainly will notice something has changed. My DC1 was 24 months, and 6 months is a massive difference at that age, but she understood really well that there was a baby growing in my tummy who would be her very own brother etc. She was cross with the sonographer at a private scan we had partly so she could be there when she was 19 months - the sonographer told her the baby was a boy, and she shouted No! First we thought she wanted a girl, but then she said "he's a baby not a boy!" - turned out she thought of someone like her when we said girl or boy, babies are babies, boys and girls are children... :o
She also asked who the baby's Mummy would be, but I think that was because she had known me as a child minder ever since she could remember and thought I was looking after the baby but that I could only be her Mummy...
So even tiny tots can have some quite unexpected misunderstandings and thoughts - do talk to her.
My DD was a brilliant big sister as a toddler, never once showed any aggression or dislike of her sibling, though she was quite naughty while i was stuck breastfeeding, transparently seeking attention by climbing etc - understandable though. She recruited DC2 into her make believe games as soon as he could crawl, and they were inseparable between the ages of 3 & 1 - 9 & 7.
If you can get on with a wrap style sling / babywearing I really recommend it. DC2 virtually lived in a sling until he was 6 months old, meaning I could be hands free and push DC1 on the swing or whatever. I mostly stuck him in the sling and carried in with dc1 as I had while pregnant, going to toddlers groups and the park etc. I liked having him close but concentrating on her, and he was very chilled as a baby.