I don't know what I am doing wrong

​

I am 18m and in summer vacation, I am often alone and introverted, my mom gets mad at me for not going out with the family involving my step little sister, step dad and herself, and she says im do nothing either way. Which ngl is somewhat true ( only if the apartment is already clean) which makes her irritated like for example, one time the leaves in the back weren't cleaned but there was like not much i would say i could count them on my fingers the amount and she told me that it needed cleaning but I was confused and told her there wasnt much which got her mad.

But, I have some reason why im not going out with them.

1. Me and my step dad always argue, each time I go out with them it ends with both of us arguing and it then becomes akward for the rest of the ride. Me and him dont have a great relationship we are more like acquainted then anything else, this is due to his actions after he found out my mom was pregnant and other stuff and i cant seem to forgive him and proabably wont because my mom and him work hard but she is always working while he isn't, she looks tired so i try to help her while he doesnt seem affected one bit which makes me irritated.

2. I feel left out, in the beginning it was me, my older sister and mom and him, now my sister moved out and my little sister was born so now its me, my mom, my step dad and step little sister. And each time I go I always walk in the back of all of them and see them having a fun time, I feel unwanted and just the extra on a family.

But, at the end of the day It feels wrong and selfish, when me and my mom talked she mentioned how selfish I was because she is in the middle of it all and I agree but when me and my step dad argue she and him always get in a argument after which then becomes a bigger problem for her. The only solution is found was not interacting with my little sister at all to not start a conflict with my step dad and not talking much.

Now I want advice in what I can do to make her understand my perspective or even how can I help her understand that everything is done so nothing is needed.