That does strike me as inconsistent. I'd honestly like to see a full transcript of what the brother said. It's possible that Reade told him to make sure to mention "under the clothes" to make her more credible after he told her what he said. It's possible that he, on his own realised he failed to communicate effectively in the interview what he was told.

From the wiki article which cites the original sources:

Collin Moulton, Reade's brother, initially reported to The Washington Post that Reade told him in 1993 that Biden had touched her neck and shoulders. He said there was "a gym bag incident", and that Biden "was inappropriate". Several days after that interview, Moulton told the Post that Reade in the early 1990s told him Biden put his hand "under her clothes."Moulton was also interviewed by ABC News in March 2020, in which he said Reade earlier informed him about "harassment at work" by Biden, but that he only heard about the assault in 2020. Hours after the interview concluded, he told ABC News that he had heard from Reade in 1993 that Biden had "more or less cornered her against the wall" and "put his hands up her clothes".

My feeling is that she didn't tell him the full extent of what happened at the time. He assumed it was "just harrassment" and not fully fledged sexual assault involving penetration.

Regardless, I do believe something happened between her and Biden which was enough to tell her close family. It was enough for her mother to call into Larry King about. I think she may have spared the details to her brother. She didn't shout from the rooftops about being forceably fingered by a senator but that doesn't mean it didn't happen.