The assignation of second partner to the ninth is based on derived houses.
For example my children in general are ruled by the fifth house. If I have three children and wish to differentiate them in a reading, then the eldest gets the fifth house, the middle child gets the seventh house (third house (siblings) from the fifth - i.e. the sibling of my first child. The youngest child gets the ninth house - again the third from the seventh - the sibling of my middle child.
I could use the same reasonning for looking at my sister and brother - my sister is my eldest sibling so if I wanted a reading solely related to her, I'd take the third house. For my brother, I would take the fifth house (third from the third or brother of my sister).
The use of the ninth for the second partner is recorded in Sophia Mason's book, From one House to another, which looks at this process of derived houses. And I used that as the source for my original answer.
However there's something of a problem with this approach in this case. The use of a 'third house' relationship works for obvious reasons with siblings but I'm not sure of the logic for second partners, who are unrelated. So the ninth could be used for my brother in law (sibling of my wife) but I'm not sure that it holds for a second wife if ever I had one.
Personally, for anybody's second partner I'd use the seventh if they were married to the second partner or in a long term relationship with them.
I've been trying to search out an actual case used by someone where there is the issue of a possible second partner on the scene before a marriage takes place or is a potential event. The only one I can find, is discussed in Lee Lehman's book The Martial Art of Horary Astrology. In this case the querent was a woman actually in a relationship with a married man and wantint to know if he would marry her. The case takes up a lot of space, as she was recurring customer, so to speak. However, whilst Lehman uses the seventh house for him (from her point of view he is a long term partner); from his point of view she uses the fifth for the woman - i.e. a romantic interest. The ninth does not come into her reasoning, despite the possibility that he might actually marry her (and thus she would be his second partner) .
The upshot is that to date I can't find the ninth actually used in analysis, despite Mason's claim. I'm not ruling it out at all but I'd like to discover a convincing reason for its use and see an example of it used in practice.
If there is an Astrological basis for the assignation then it's possible to turn the chart and do a full reading, as if the ninth house were the first house of the prosepective partner - I used this approach above and it's a valid technique.
Thus if you was to do a chart for your sister-in-lawss cleaner, You could use your own chart, take the ninth house (seventh (marriage partner) from the third (brother) to get your sister-in-law and then count to the sixth house from that (the second house in your chart) which would be the house of her servants. Using that as the first house of a chart, proceed as normal.