There is no feminine form. Senator is the word for both men and women equally.
It is Senator.
The feminine form of ami is amie. The feminine plural is amies.
The feminine form of bajo is baja. The feminine plural is bajas.
The feminine form of alumnus is alumna. The feminine plural is alumnae.
Mistress is the feminine form of master. It is already in feminine form.
The feminine form of a baron is a baroness.
The singular is senator and the possessive is senator's (one senator).The plural is senators and the plural possessive is senators' (referring to more than one senator).
The possessive form of the singular noun senator is senator's.Example: I got the summer job as a senator's intern.
The feminine form of charmant is charmante. The feminine plural is charmantes.
The feminine form for host is "hostess."
The feminine form of benefactor is benefactress.
The feminine form of "mauvais" is "mauvaise."