I am adopted and was searching for my birth family and found a cousin and since then I had someone in the family helping me with the search. I found someone who I think is my half-sibling. She did a DNA on Ancestry and she came back as a 1st cousin but our CM's are 1286. We can't be 1st cousins because her father had one brother but he was a child when I was born and their were no other brothers. I have done a lot of research on this family and I found out my sibling's father was where my birth mother was during the time I was conceived. We also resemble each other quite a bit. I know CMS are a range of numbers but this is so close to a half sibling I feel sure she is.
There is no such thing as a cut-off, just a range of probabilities.
The Shared CM project tool shows that half-sibling is one possibility with that amount of shared DNA, but there are others that are equally possible. If you've done a lot of research on the family, the WATO tool can help you model the possibilities and assess which is most probable.