I think if TV, games, days out dont work then you need to address the one thing that WILL work - dad. He is the ultimate sanction.
SO if they hurt the little one, dad could say, "Im sorry, I have to keep LO safe, and to do that I will have to take her to the park. Sorry that you cant come with us, because I have to make sure little one is safe, and you arent helping me keep her safe, you are hurting her."
Have a list of nice things that he can do - pop to the shop for icecream, softplay, park, swimming, walking the dog, visiting granny....who knows.
The key thing is that the LO gets dad and all the attention when hurt, and the older two lose his attention, while he reiterates the message that he has to keep the LO safe from being hurt. As a PP said, you can even divide and conquer - if one hurts and the other doesnt, then the kind one gets to go out.
This does require you to pick up the slack, but as a logical consequence, tied completely to their behaviour, it might work. You will not need to do it forever. ANd you need to use the same language, both you and dad.
ALso, he has to bevcome immune to the guilt tripping and the tears. He can say his piece very sadly - even throw in "I really wish I could take you, it makes me sad that I cant, but I have to keep LO safe and you keep hurting her/him"
You could even let them know the nice things you are planning for the weekend, so that they have to make choices about what they do - hurt LO or go to Macdonalds for icecream with dad.