Wait... What? Whose the Beom Taejoo again?

ml's father

So, beom taeha iz the main lead right? What's the name of his brother then?

Taehee

i think his name is beom taehee

so:
beom taeha ML
beom taehee his big brother
beom taejoo their father
this new old lady is either taehee or taeha's mother (I lean more towards her being taehee's mother)

I appreciate the help, thanks