Congratulations on the imminent arrival of your ds.
My elder sister and I are both very pale skinned and freckly with dark hair and look very much like our father and his Irish family. My younger half-sister is half-asian and has dark skin and beautiful asian looking long dark hair, she is the image of her (asian) aunt, so much so that she has been mistaken for her on occasion.
She looks nothing like us, but very few people commented on it when we were growing up and those that did, didn't bother us in the slightest.
Her dad was never a part of our family, so I would have thought it quite likely that people would have asked if she was adopted. To be honest, even if they had asked we would have just said, "no she is my sister" or something similar.
My husband is one of three from the same parents. He is blonde with blue eyes, mid-height and skinny, his brother is short, stocky, ginger and freckly and his sister is mid-height, absolutely gorgeous with dark skin and dark eyes and hair. To see them together you would never guess they are related, let alone siblings only two years apart.
Please don't worry about it. Families come in all shapes and sizes these days. All that matters is that you are a family that love each other, it really doesn't matter what other people think/ask/say.
Probably worded that really badly as not feeling to good today, but hope you got the gist of it anyway.