This is one of those Questions with as manyt answers as there are pebbles on a beach.
Everyone will give you an opinion, but it is just that - based on their own experiencce which may be good or bad.
I am the youngest - two older sisters, now aged fifty-six and sixty-three. They were fine growing up, and are very close now. I was not close to either, and haven't seen them for years, even though we all live close to each other - long story - so i can't point to siblings as a happy experience for me.
I inherited my two older daughters, and they were fourteen and twelve when their baby sister arrived. The older were fine as children, and are very close as adults - they are now thirty-five and thirty-three. Our youngest was, in many ways, and 'only one' growing up because of the age gap, although her sisters adored her, and she them, and now she is twenty-one, she is especially close to her older sister.
i think your daugter will be fine, with or without a sibling. Let's face it, we only grow up with our own circumstances, which are all we ever know, so the idea of transferring the experiences of others - even her parents - onto your little one is pointless.
If you love her and guide her, there is no reason why she will be a 'brat'.
As with anything to do with children - follow what you and your husband want, and close your ears to the 'advice' of others - in this situation, advice is only good for those who give it.