Who is the most forceful member of the Merrick family?

One could argue that Mrs. Merrick, the late sculptor's mother, is the most forceful member of the family. She dominates everyone around her with her violent, domineering personality. For good measure, she is also a drama queen whose histrionic display upon seeing her son's coffin convinces absolutely no one of her grief's sincerity. Generally speaking, the sculptor's hometown is trapped in a state of paralysis. Although Mrs. Merrick is as much a part of this town's life as anyone, she does at least give the impression of being a full-blooded human being, which is more than can be said for most other folk in Sand City.
On the other hand, it could also be said that Laird is the most forceful character in the story as a whole. It is his bitter, passionate tirade toward the end that administers a needed dose of truth to the assembled mourners. But because Laird's outburst can be easily dismissed as the ravings of an alcoholic, it is unlikely that the townsfolk will pay much heed to him and face up to the truth of what Sand City does to free, creative spirits. Instead, they will almost certainly go on much as they have always done, with Mrs. Merrick continuing to stand out among them as one of the town's most forceful characters.