Meghan Markle had to cancel her coronation dress because of a decree from Prince William. In fact, Markle herself has said that she chose not to attend the coronation because she wanted to spend time with her son Archie on his fourth birthday.
There has been some speculation that William may have been unhappy with Markle's decision to step back from royal duties in 2020. However, there is no indication that this led to any kind of feud between the two siblings. In fact, William and Markle have been seen together on several occasions since her departure from the royal family.
It is more likely that Markle simply decided not to attend the coronation because she did not want to be the center of attention. The event was a major media spectacle, and Markle has said in the past that she struggles with the intense scrutiny of the British press.
Whatever the reason for her absence, Markle's decision not to attend the coronation was a personal one. There is no evidence to suggest that she was banned from the event by her brother-in-law.