I think the whole thing missing from this is we can speculate how Serena is until we are blue in the face based off a few snippets that we have seen on TV and what a few people say about her. However, unless we are in her life on a consistent level it really is hard to say who the real Serena is. Sloane can think Serena is this awful person for not signing her poster, but if you ask me getting to wear a dress made by Serena and pose with two tennis legends for a picture is better than a signed poster. Serena has never said anything awful about Sloane. In fact, she was very complimentary and didn't use injury as an excuse for Sloane's AO win. She has had positive things to say about Sloane and her game. I mean Serena called her a potential future number 1. What more does Sloane want? Does she want Serena to pose as her and win a slam for her? You are competition. Once you beat her, she is bound and determined to not let it happen again. She sure a hell isn't going to like you after it. Sloane=Maria S. after her Wimbledon win over Serena that simple.
I think where race comes into play in this is Sloane probably feels Serena should have her back even more as a fellow black woman.
The one thing I hope doesn't happen as a result is older players turning their backs on younger players for fear of drama.