Feeling like a bad mommy

First I wanna start by saying I love this app so much you guys don’t know how much you help me and remind me I’m not the only one that’s going through stuff as a mom. Y’all are wonderful moms so I come to y’all before my family lol

My baby is 2.5 and she is so “bad” like she doesn’t listen , always telling me no , she throw stuff when she doesn’t want it , she gets impatient sometimes. she also like a sponge she copy’s everything someone say including curse words which I keep telling my siblings to watch their month. But her behavior is more she doesn’t listen and so demanding I would say she is “spoiled” but of course I treat her like she the princess she is and so does my mom but my mom got her into this phase where she feel like she can take someone sit or whatever they have in their hand cause my mom would take something from someone and give it to her basically giving her whatever she wants. She is also basically Potty trained , she still have a few accidents if she can’t get her pants off but anyway my family makes me feel bad because my niece is such a good kid she is 3 which my niece have other siblings and mines seems to have behavior problems but she’s the only child. They stay coming for my parenting like I do my best and I try gentle parenting it’s hard tryna be gentle to a child that’s not gentle. She like sweet and sour she have her days when she the sweetest baby ever and so kind and gentle and very careful to her baby niece like she is naturing as well and she cares if people cry she tries to comfort them she also have manners she says yes please and thank and you welcome and excuse me when she farts lol but Idk I just feel like I’m not doing a good enough job. Idk what to do about her bad behavior sometimes I think something is wrong with her because my niece is the perfect child. Sometimes I pop her but I don’t like it at all. Please help me ladies. Is it because she isn’t in school? She really disobedient sometimes and sometimes talk back.