Soft YTA

I understand basic Mandarin and speak two other languages fluently, including English. As a non English speaker, I would try to make everyone (at workplace/home) feel comfortable and not excluded by speaking English. This usually make people feel at ease and let their guard down. When someone purposely/suddenly switch their language while conversing in front of you, you bet that it is something they don't want you to understand/include you in conversation.

For example, I am fairly new in a workplace years ago. 2 male Chinese colleagues and I was having brief conversation in English. Then out of nowhere, one of them start switching language and talk to the other colleague in mandarin about something inappropriate (borderline sexual) in regard to me. They don't know that I can understand them. I kept my cool without any facial expression. The ball to talk shit and inappropriate when I'm standing there, had me flabbergasted afterward. I then report the incident to the upper level and they both get reprimanded.

2nd example, I have a female Ukranian colleague who confess in me and ask if her Chinese coworker are talking shit about her. This happened when I was not around. While in a lunch gathering, everyone speak English and then suddenly her Chinese colleagues start speaking mandarin in front of her. She just sit quietly, too shy to even ask what they are talking about. She feels awkward the entire time. I told her too bad, I wouldn't know what they talk about (if it is just personal or work related) since I'm not next to her. But it definitely make her walk out every time her colleagues speak a language she doesn't understand.

Honestly, your kids just want to communicate with each other without including the stepson. It's the truth. They don't even make the effort to even teach the boy any Urdu word if they want to include him in the conversation. You would be a soft YTA for not making the home environment inclusive when stepson is making effort to be around the other 3 children. It is your kids choice and at the end of the day, step siblings will just be step sibling. Nothing will bond.