Im in the earlier chapters and i have a doubt does the mc also sleep with the old guy?

Nope! Jus his brother did, the only one mc sleeps with in story is the ml (minus his past relationship which was also not with an old man)

Then the lingerie thing was his brother and the ml was just misunderstanding? And can you please explain the video clip i didn't understand it like does the mc enters after the oldmans death?

Yup! So basically MC's brother was having sex with the old man who ends up dying cause he's so old, he then calls his older brother (our mc) to help him, our mc then puts on the lingerie and masturbates so he can take the blame instead of his little brother (who's an actor)