I'm a bad mom

I cannot seem to have a pleasant conversation with my brother. It always ends up wrong. Yesterday it was about his 2 year old son, P. They had him when they had been told they probably never have children. And after 13 years of marriage, and totally unexpected, they had P. And to me, they have so many expectations with this child... they might just not leave room for him to grow. He's supposed to be the perfect child. To me, it seems like they just have a new appliance at home and follow instructions one by one. He's not just a kid, he's a museum object. And they're the only ones entitled to handle him. I was at the beach yesterday with my brother, the girls and P. I told my brother to go swim a bit, I'll watch over the children. He said no, he doesn't trust me. "What, do you think I'll let him drown?" i asked. Well, he just wouldn't go two steps away from his child. "So you'd never let me take him for a walk with the girls, right?" and he admitted he wouldn't. Both he and my SIL feel I'm careless. In their eyes, it's a miracle my daughters are alive, considering I seem to place them always in danger! You see, I don't run after them all the time. They find it shocking, because they always have a pair of eyes on P all the time (except at night), and I'm not exagerating. I've let my daughters sleep over at their home, but they will not let me have P without their watching. So all this has put some strain in our relationship. I'm fit to look at him, to admire him, but not to take care of him. And I'm mad. They find it surprising that my mom doesn't volunteer to take care of him (they would trust her more than me). I know my mom doesn't dare do that, because she's so afraid something might happen to him and they'd blame her! Last sunday my mom offered to take care of him, but SIL said no because there's a small inflatable swimming pool. My mom said "don't worry, I'll be watching" but she said no "No no, thank you, the doctor said kids can drown even in very shallow waters..." Anyway, I'm being bitter, probably, but I can't help it. We're supposed to look at him, admire him and say how cute he is, but NOT treat him like a kid. It's like handling a very expensive vase. It's not like that at all with my other nephews... and they manage to make me feel I'm such a bad mother... I should be giving my kids organic food, change their clothes immediately if there's so much as a small stain ... my brother even accused me once of letting Cristina get colds! I have the feeling if something happened to the boy, their marriage would break.