When you say "je n'ai pas de soeur" it means "I don't have sisters" but if you twist it, it means "I have zero sister". If you have zero sister, you don't need an "s" at the end of sister (as "s" at the end or words is for plural). For that reason, I prefer "je n'ai pas de soeur" mais but it would not be a major problem to write "soeurs" (with an "s") because if you don't have 1 sister, you automatically don't have several sisters (plural). Do it make sence for you ?

When speaking, both sentences will sound exactly the same. We add "s" to a word when it is plural but we don't pronounce them. It's only a convention.

I don't have any sisters = Je n'ai pas de sœur / Je n'ai aucune sœur