Possessive of my kids

Me and my kids (one is 5yo and the other is 4mos.) have been living with my in-laws while my husband is away, working. I really don't mind my MIL and Sis-in-law helping out especially with the baby, but I'm really feeling very possessive especially when baby is with SIL, acting like she's the mom, taking baby from me even when I don't ask her to.. Also, she's been spoiling my 5yo by buying her with so much stuff (hello junk food!); I really do appreciate it but I don't know why I'm feeling paranoid that they, especially my baby might become closer to her, she's able to make baby laugh more! :( I don't have problems with my own mom & sis but when it comes to my in-laws, it becomes a problem to me. We don't have any problem with each other but I don't know... I really hate this feeling. I don't know what to do. :(