Who is this chairman? The Grandma, taejus aunt was talking to. How is she related to them....

I cant tell the exact though i think they're lending company legally and doing something illegal like land/property mafia because they're always talking about construction project and how it took so much effort to get that property for them

So that chairman lady is the one who put the construction contract up for bid that taeju's family company, eunhwa (the company that kidnapped euihyun), and a couple of other construction companies all really want. That's how construction projects are usually decided irl too (a project is wanted by a company and then construction company's bid w their design and say how much they can get it done for).
All the companies are corrupt but not necessarily mafia related like youngdo (or whatever his name is) said earlier to euihyun's dad. Taeju's aunt made a deal in earlier chapters to get the construction contract but had to reneg on it for now as part of getting euihyun back.