My Sister-in-law "borrows" Money From Me Without Her Husband's Knowledge.

1. I just came from naija and she knew I brought money for my upkeep. While I was staying in their house for the first few days. She asked me for $60 to be returned by the following week. it's now over a year.
2. more than a year later, I had to come back to live with them so I bought some stuff for household use. Her husband gave me a check to offest the bill but I initially refused. she later took the check from me without the husband's knowledge.
3. A guest came from naija and we all went to the mall. She saw something she loved and asked that I paid for it till we get home. it's now over 2 months
4. Today, she just asked me if I have some cash on me and I said yes. She asked that I give her some money for some need till next week.

My concern is that I am staying in their house FOC so I don't mind putting something down in lieu of rent. But I don't like the idea of a woman taking money from a man that is not her husband. Apart from the ethical issue of borrowing without paying back, it is a sign of longthroat and, with my upbringing, I believe that women that have such attitude have tendency to engage in extramarital affairs (either by accident or delibrately). I don't want anything like that around me. I have enough of my problems/temptations. I don't want any wahala from that angle. Her husband is my own wife's senior brother. And we (my S-I-L and myself) are almost age mates. Again, she tells me of her escapades in her younger days and of course I can't tell her husband. All these are just so stupid, as far as I'm concerned, because there's not supposed to be any secret between a man and woman that are not married/related or that do not have a professional relationship.

What do you guys think?