Dear Iyanda, please what should I do about the unbecoming relationship/friendship that is brewing between my husband and my immediate younger sister?

For the records, my sister is an undergraduate and comes to stay with us anytime she is on holiday. Things have been that way since her first year in the university; and now she is in her third year. My husband likes her a lot and I didn’t see anything harmful in that until my attention was drawn to some strange things last month.

For instance, I was going through my husband’s trousers and shirts to be sure no documents or money was left in any of his pockets before handing them over to the laundryman when I saw some payment receipts. The details revealed some payments made for the purchase of ladies undies, bras, skirts, lipsticks and the likes. I knew he didn’t buy any of those things for me recently, so I sat down to add one plus one together and decided to go to my sister’s room while she was not at home. To my surprise, I saw some of those things on the receipt where she kept them in her room. The items matched the same ones on the receipt.

I waited for a week or more to see if either of them would open up on those items, but none did. Meanwhile, I noticed that they were spending more time with each other even while in the home. In all honesty, I haven’t noticed anything strange beyond the receipt and the fact that they now spend more time together.

At least I have the receipt as the evidence and proof that something is going on or about to start brewing; but how long do I have to wait before I confront the two of them? Should I confront my sister first or just call the attention of my husband to what I saw on his receipt?

I am afraid something bad might happen to my marriage if I don’t act fast. I am really scared and don’t know how to go about this; please you guys should help me out.