How do I deal with step kids?

So I am in a relationship with a guy whose wife died 2 years ago, and he has 2 kids a girl and son. His daughter he doesn’t care about and she hates him. His son now is attached to him at the hip, it was so bad he would make his son sleep with him every night and stopped his kid from playing with friends because he didn’t want to be alone. Now me and his dad had been together for some time, Now he refuses to discipline his kid, excuses everything that he does for example even when he is blatantly rude to kids, his family, and even adults his dad is in denial that anything he does is wrong. He even is rude to me all the time, and he blames me for everything that goes wrong. Also he makes jabs to me like ohh I miss my mom, I wish she was here, I hate life now and I take them as direct jabs towards me. And his dad can do no wrong at all. What can/should I do?
How do I deal with step kids?
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