having problems with brother's friend

My family knew that I was looking for a new pet.  I wanted a cat or a kitten and had been looking in shelters and in ads for a pet, hoping to give an unwanted animal a good home.  I finally found a 3 year old super fat and fuzzy faded orange tabby cat and brought it home. Named him Puss in Boots because he reminds me of the character in Shrek 3, that fat!  Well my brother called me up and explained to me that a friend of his was looking for a home for their 3 month old Bengel because:  the wife and daughter had allergies, because he clawed everything, wasn't cuddly, they didn't like having to have a litter box in the house and basically did not want not keep him. 

I went to see him, took one look at him and fell in love.  He's beautiful, tall, dark and spotted, thin build with long legs, just the opposite of my fatty Puss!  ((they say he's a Bengel and he does rather look like one, but they paid only $100 for him and I know Bengel cats are very expensive)  It has been a joy having these new kitties in my home.  I have had them both for about 4-1/2 months now.  They are both doing very well, are healthy, happy, very frisky, playful and very, very cuddly lovers.

A couple of weekends ago at a family outing my brother's friends were there and the wife asked how the cat was doing.  I told her very well and told her thank you and assured her that he was in a very happy hope and being well cared for.  She cried a bit (I need to add that she was intoxicated...) and told me that she wanted him back.  Her husband laughed it off as her being drunk, but over the course of the day she said it about 3 other times.  Even her daughter apologized for her, she's 16.

Last night at gathering at MY place (when we have family get togethers it is usual that we bring a friend or two to each other's place, it's always been this way) my brother's friends came over.  The wife (again intoxicated) saw the cat walking around, snatched him up and collapsed to the floor crying in a very exagerated way...  The cat got scared, bit her pretty good, scratched her legs and took off to hide under my bed.  after that she was walking around moaning and crying out loud about how sad she was, how she wanted the cat back and made a big scene.  My brother and I told her she needed to leave, she said she was going to, but she was taking the cat back.  She went into my room and started trying to get the cat from under the bed!  I told her 10 seconds to get out or I was calling the police.  (my sister actually made a grab for her hair, but we stopped it from getting any more physical)  She left, yelling and crying that she was coming back for the cat.  Her husband and daughter apologized and told me not to worry about the cat, it was mine and they even thanked me for taking it in.  Crazy woman on the other hand was yelling that she had his "papers" and was going to get him back.

I have invested time and money into this kitty, but more importantly, I LOVE HIM.  I am angry, yet at the same time it feels ... weird keeping him, like I've stole him from someone.   I have had him neutered, microchipped and well, he's mine!  If she comes back, I don't know what I'm going to do.  Should I offer to pay them something or just write her off as wacky and let it be, since her DH said he's my kitty? 

Coincidentally, I have had 5 hang up calls today from "unknown caller"...  hmm.