How can I deal with this like an adult?

I live with my boyfriend of 6 years, and just recently we had his mother move in with us. She has been living with his sister and that is longer working out. She doesnt have a car or a job and lives of a fixed income of $500.00 But thats besides the point I'm getting at. Anyways, we (me and my boyfriend) got a kitten he is now 5 months old. She kept saying, "the cat will end up getting attached to me cuz i will be with it all day. Mandy's cat ( my boyfriends sister), got attached to me too, so yours probably will do the same.", not meaning harshly just telling us. Well lately I get so P.O.ed every time she starts playing with him when I am at home and its like a feeling of jealousy and it overwhelms me!!! I get all red in the face every time she says "kitty kitty kitty" and plays with him! I know its sounds childish and petty but i just want to smash her face in everytime! Its like she WANTS the cat to get attached to her on purpose! I am 27 years old and need some advise, HELP!