It sounds to me like your brother may have just taken the wrong approach. Perhaps involving that sister less by means of active participation, but just exposing her to the creative process and using her as a consultant may have improved that outcome. I don’t figure you or your brother are expecting advice, but generally I find that it’s best not to try and dominate someone else’s interest as a means to interest them.

The toughest part about enjoying TTRPGs is finding a DM that lets you exercise your variety of creativity. If your only experience is with your brother as a DM, it could be you just don’t necessarily vibe with his style of doing so.