This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
嬁 combines 女 (woman) with 登, which likely contributes the sound. The exact historical development is uncertain.
A woman 女 beside 登, a shape associated with climbing or ascending. The meaning is uncertain, but the shape suggests a woman rising or stepping up.
For トウ, imagine a woman climbing a tall tower: tower -> トウ, and she ascends step by step.