Would a female Jedi be referred to as Jedi Mistress instead of Jedi Master? Likewise would a Sith female be referred to as a Sith Lady instead of a Sith Lord?
In current canon there are no female Sith Lords (Asajj Ventress was affiliated with the Sith as an apprentice of Darth Tyranus, but was never given a title like Sith Lord or a feminine equivalent). There were many female Jedi Masters, and they were called Jedi Masters rather than Jedi Mistresses (e.g. Shaak Ti and Adi Gallia).
Female Jedi and Sith generally use the titles Jedi Master and Sith Lord in Legends, too. The novel Dynasty of Evil includes the following explicit reference to a female Sith using "Lord" for a similar title, "Dark Lord of the Sith":
The Master will train his apprentice in the ways of the Sith, until one day she must challenge him. If she proves unworthy, the Master will destroy her and choose a new apprentice. If she proves the stronger, the Master will fall and she will become the new Dark Lord of the Sith, and choose an apprentice of her own.
There are a few exceptions among the Sith, however. For example, Lumiya used the title Dark Lady of the Sith rather than the normal Dark Lord of the Sith.
Collins English Dictionary lists "a highly regarded teacher or leader whose religion or philosophy is accepted by followers" as a meaning of "Master," with no mention of gender, though it does specify male in other meanings. The general feeling is that "mistress" is not an acceptable feminisation of "master." "Lord" is used in a gender-neutral way in the House of Lords, if they are an authority on anything.
Jocasta Nu (the head of the Jedi archives and a former member of the Jedi council) goes by the term Madame in the second of the preview movies, as is referred to as Madame Jocasta Nu in the script. She is listed as a Jedi Master on Star Wars wiki, however. I think that the title itself is unisex, meaning that both men and women can hold the title "Jedi Master" however when addressing a female Jedi Master, one may use the term Madame to show respect or be indicative of a high rank.
There have been instances where there was a female Sith that was referred to as a "Sith Witch." However, there were also female sith that were not. So if that helps answers at least part of your question... I have posted a question about more details on the matter. Is “Sith Witch” a title, or a general term for female Sith?