I am a new member and this is my first post, I am going through a challenging battle with myself pertaining to my current relationship.

I have been dating my now Fiancee for about 4 years now and we are both 30. She has 2 children, Girl is 11 and the boy is 5. My main issue is how my SO handles her issues with the 5 year old and the bio dad. She pretty much had to take him to court before we met so he can pay child support, He did not want to give a dime on his own for his children. There has been many times in which we have made plans when they are supposed to go to his house but some way some how when that day comes up, Something comes up. Which really means he did not ask his mother if she will be home because he works weekends and when he is off he goes to play volleyball with his new girlfriend whom he cheated my now Fiancee with. Something always happens so he doesnt have to spend time with the kids, The girl has noticed that and recently wrote a very firm letter to him. His way to get around any negative things with the kids is buying anything and everything they want which as children they fall for it. I was raised in a strict household especially when it came to cherishing what you have and what you got. I know my folks struggled to keep afloat with three children and im blessed till this day my parents are still alive and together. They have been married for 45 years which is unheard of these days. Anyways, When i bring up the issue about how he likes to throw wrenches into our plans she just says "Hey life happens its something that is beyond our control and we just need to reschedule." I am ok with that when its something that CAN be rescheduled but not with things that can not be. A concert or a vacation overseas for a weekend which ill lose 50% of the cost because of this person. If i say anything about him because i feel that he is just trying to screw us over she automatically jumps into defensive mode saying "Its not necessary to talk about him like that or his family like that despite all they do." He would ask other people to pick up the children but never has a way to bring the kids back so he puts my fiancee in a situation which she HAS to drive 45 mins to pick them up. 

The other part is discipline when it comes to the boy. When i met him i knew he was going to be a force to be wreckoned with, I told her "Hey, If you do not start correcting his behavior now he will be worst." The line was always "He's little and will grow out of it." Hes disrespectful, Talks over other people and does not stop until the attention is on him. He knows to say excuse me to say something but he will continue saying it over and over until he is acknowledged. He does not listen to his mother or anyone else in the family. Back then i was strict and she asked me to discipline him i said are you sure? she said yes. So i put the hammer down and he did a full 180 then i stepped down and he has steadily been getting worst. She does not discipline at all, When she blows her top thats when she puts her foot down. It shouldn't get to that though there should be some type of system to combat that behavior. She does 5 min timeouts or 10 min timeouts and at times hes like "Mom can i ask you something?" He would go to her and start a conversation and the timeout is over. I call her out on it and she just shuts down or feel some type of way about it. Her whole family including herself would use my name to instill fear in him. I do not want to be that person, I do not want him to hear my name and say "Oh shit im screwed." I spend time with him when hes not out of control and we play. I make up weird games with his toys like shooting a blue ball from his firetruck into my fiancees boot to win points depending on distance and he enjoys things like that. Recently he developed an interest in what i like which is anything with an engine/motor. I am an Auto Technician and i love taking things apart and putting them back together. I watch videos about restorations and the like, He has been asking me all the time to watch it with me and i do that with him. But! When we are not doing that hes just throwing things around, Whining, Yelling and talking back to his mom. Hitting his sister and then lies about her hitting him first when ive personally have witnessed him hitting her first. I confront him and he just stays quiet and thinks about a way to lie to me, I tell him dont lie because the punishment will be worst and he just doesnt speak. I take away toys, the screens and anything he enjoys for a couple of days or so but my fiancee of course gives him her phone so he can be quiet and let her do her thing. We are not on the same page when it comes to discipline and its a wedge between us. When he goes to his fathers house he is a demon over there and then brings that here to our home. He thinks he can do whatever he wants like he does over there, There are times in which the girl calls me to talk to him because he only listens to me. Not his father, or grandmother. To my understanding he only listens to his grandfather when he is over there and the grandfather is barely there so pretty much free reign. When i mention anything about discipline we always have a cordial back and forth but then she shuts down and ignores me for 1-2 days. When ever there is a problem she shuts down and does not tackle it head on and hopes the problem just goes away on its own. I dont like that, If there is an issue then talk about it, Do something about it, Lets come up with a plan and lets beat this thing. But, She stays quiet and then she gets so emotional to the point that she cries by herself and wont say anything about it because there is nothing that can be done. All the times in which she said there is no solution i have found one but she refuses to follow through. 

I guess its a lot for me because i was raised in a structured home with rules and if you did something wrong you get the death stare. I love my parents and yeah there are things they did in which i am not happy with but it made me who i am today and i am very greatful for it. 

Am i just being a baby? If i am then please tell me, I need honesty and suggestions about what i should do. Slowly but surely im developing resentment because i know i can help and she refuses to ask for it and just drowns in her own mind. Wont go to therapy so she can let it all out, She has issues with her immediate family and her brother passed away 6 years ago, She has gone through a lot and i feel her not letting that go is affecting her as a person today. Does not let her grow into the best version of herself. Just letting situations control her life.

SO SORRY FOR THE LONG POST!!! WOW!!! My humble apologies i didn't thing there was going to be this much to say.