wife of mother's brother
妗 refers specifically to the wife of one's mother's brother, a precise kinship term. Its meaning is narrow and relational, with no broader conceptual center.
妗 combines 女 (woman) with 今, which likely contributes a phonetic element. The exact historical development is uncertain.
A woman (女) who is connected to the present (今) family: your mother's brother's wife, a relative you might see now and then.
For セン, imagine your aunt sending (セン) a message to the family: セン sounds like 'send', and she is the wife of your mother's brother.