I wouldn't feel bad, we do the same. I save birthday, Christmas, could benefits for our daughter and my partner saves any birthday/ Christmas money SD gets from his side. SD already has thousands in a bank account that her mum has control of, so my partner doesn't feel the need to put any extra away for her x
Nope I do the same and I don't feel bad. I save child benefits money for both my girls and move it to their junior ISA every month, my step kids mum gets more money than we do in a month via benefits and child maintenance (which she shouldn't be getting as they're here half the time!) that if she wanted to save for her kids then she could, I doubt she would though. The kids never miss out on stuff when they're with us which is 50% of their time and he always sends extra money for birthdays/Christmas/uniforms etc
Don’t feel bad at all. Save it all for your boy. She has two parents who are more than capable of doing this.
Her mum receives the child benefit for her so she should be saving it for her. I save for our two but we don't save for my partners child because we spend lots on his when he comes at the weekend x