Start as soon as possible is not composiry for you to know the sex of the baby you can buy 5 unisex cloth and as soon you deliver before they will discharge you your sister can help you to buy the baby clothes. Don't forget that this is your first time you need many things
I advice u start buying now because u might not av d strength too in d latter stage of pregnancy. U can do more of unisex wen it comes 2 sleepsuit, bodysuits, baby accessories like bathing things, cap, mittens, towels, flannels, wash cloth and all that. Afterall that's wat d baby will need in d first 5weeks. Other stuffs like outing wears can b bought wen u know d sex of ur baby