I need some help (short)

I currently live with my boyfriend and his family.

His niece is quite fond of me, she loves to hang out with me and just be my best friend.

My only concern is I recently lost and item of mine and I’m not sure where it exactly went. I’ve retraced my steps. I’ve asked everyone around. My family as well because I visit them from here and there. I looked in our entire room. I’ve looked in our bathroom. I’ve completely redesigned the room and looked absolutely everywhere. I can’t find it anywhere. The reason I’m kind of going towards his niece(she’s 7) because I left my phone in the kitchen for 5 minutes and it ends up missing. I think it’s in my room and it ends up being under her shirt and she drops it. I’m dumbfounded like hmmm maybe it fell from the top of the counter but her moms like what were you doing with her phone. The niecey says I was gonna give it to her but if she was I was right in front of her searching for it why didn’t she? She’s taken my phone a few other times as well. My boyfriends confronted her about it as well. Anyway she’s a pretty smart kid for her age. I don’t want to accuse because I do live in their house that would be so rude of me please help me. We were asleep from 5 to 10 or 11 when I woke up and by that time they were gone and the niecey comes in and out of the room as she pleases. I can’t afford to buy new ones atm.

She also slaps my boyfriend in the face occasionally and me in the arm. He doesn’t want to tell her mom which I believe he should. I don’t know what to do so I just say no no no no not nice kiddoo. I love kids I’m a gentle soul. I have 6 nieces and nephews of my own. I’m really at a loss please help.