Thoughts please

My sister, her kids, and her boyfriend (who is not the father of either child), and I live together. We live together because my sister works part time and can’t afford to live on her own so we live by splitting all of our bills by thirds. I used to pay for my own place by myself but anyway, I pay 2/3 of the bill and he pays 1/3. Personally I feel like since it’s serious enough that they live together it should be serious enough for him to help her with money since he knew her situation but that’s for a different day.

Is it appropriate for him to yell and hit her kids? The first time I saw him smack my nephew I lost it and he’s never done it again in front of me and my nephew who is 5 years old tells me he hasn’t done it since and will tell me if he does. My niece who is 3 years old he adores. Always carrying her, in the bedroom with her, buys her things etc. My nephew isn’t even allowed to go into his mothers room because of the boyfriend. I just don’t think it’s right to play favorites...I think he should be equal with them both and I strongly believe that they shouldn’t be yelled at by her boyfriend who doesn’t do anything for her or the kids. My nephew gets yelled at harshly for jumping on a bed or making a mess but I think it’s normal for a 5 year old. Idk...my sister and her boyfriend act like I’m wrong for voicing my opinions but i just think it’s wrong. What do you guys think?

Some background:

They’ve been together for less than a year. However, we grew up together with him so we’ve known him our entire life.

My sister and I are close. We have lived together our entire lives with only a few years of living on our own. (She was married to my nephews father - once divorced moved in with me) I have been apart of my nephew and nieces lives since the day they were born. I help my sister with anything for them. I love her kids very much.

Some more information and answers to comments****

In my opinion, I believe it is my business as I have financially, emotionally, and physically cared for my sisters children when she can’t or could not. I understand she made them but as a decent human being I can’t find it within myself to not help her when I have the means to do so and I love my niece and nephew as they were my very own. I only have their best interests at heart.

Also, I have worked in the social worker field since I got out of college. My very first job was working for department of child and family services/child protection. Sometimes I feel being exposed to that has made me more sensitive to what could happen as I have saw/heard very harsh and cruel things being done to children by their mothers, fathers, partners, family, friends etc. I don’t believe in smacking kids. My nephew is a very hands on, rough boy but when I say it’s normal for a 5 year old it is...but it doesn’t mean we let him get away with it. I believe talking with him and processing what he is doing and taking something away such as his Minecraft time for the day is punishment enough not screaming and or yelling.

Im sorry but my nephew has a father that is in his life and although I don’t oppose to a step father disciplining him as well if need be - her boyfriend isn’t a step father. He’s just her boyfriend that lives with us because of my sister. Now if he took more of a step father role then I’d be more susceptible to it if he went about it appropriately and correctly but he does not. He doesn’t help with anything.