My half sister (Maternal) share 1673cM across 50 segments. Cousin #1 to me shares 994cM across 40 segments. Cousin #1 shares 1056cM across 46 segments. Her maybe father is my mothers brother. All of my other Cousin's on Maternal side are in the 700-800cM range.This is odd as there has been rumors for decades that Cousin #1 and my half sister share the same father. My mom and Cousin #1's mom (no blood relation) may have been impregnated by the same man. If they are half siblings would Cousin #2 and I share any DNA or are we all just 1st Cousins
You say, "Cousin #1 to me shares 994cM across 40 segments. Cousin #1 shares 1056cM across 46 segments." Is the second Cousin #1 supposed to be Cousin #2, who is mentioned in the last sentence?– shooverMar 22 at 5:10
According to DNA Painter, 1673 cM is a half-sibling relationship, which you seem certain about anyway. All other relationships described fall within the shared cM range for 1st cousins (396 - 1397), including 1056 cM, which is what I think the amount shared between your half-sister and Cousin #1 is (not clear from original post). 994 cM is also well within the range for 1st cousins, and both preclude any half-sibling relationship between yourself or your half-sister and Cousin #1 (or #2? - again not clear from your post). You are all 'just' cousins.
Your other maternal cousins in the 700-800 shared cM range with yourself are still more likely to be whole 1st cousins, but half 1st cousin relationships cannot be ruled out.