He has no fear?!

My two year old used to cry when he was told ‘no’ then we had to live with my parents briefly while we looked for a new house. My mum can’t cope when he cries and babies him, so when I told him off, she would butt in and make him laugh or give him food or whatever to stop him crying. Now if I say no he bursts out laughing, even if I raise my voice.. apparently even more hilarious. What the hell do I do to correct the damage she’s done?? Or is this normal toddler behaviour testing boundaries? He literally has no fear so will charge into doing something dangerous and I will say no and try and stop him or say stop and he just laughs and carries on, climbing on a chair or whatever he’s decided looks fun whilst I’m feeding his baby brother 🤦🏻‍♀️🤦🏻‍♀️🤦🏻‍♀️. Help??!!