Sister Stealing Entire Estate Of My Deceased Father, Please Help!

Location: Illinois.

My father died without a will. He had a lucrative business where he lived in Illinois, including city/county contracts. My half-sister, who he informally adopted (from my mother's first marriage) is on the LLC for his business and every single bank account -- about 6 accounts between personal and business. He lived with her for the past 10 years or so, and he paid the entire downpayment on her house, about 20% of the initial price -- he's not on the title. She's claiming she is in charge of everything. She has lied about how much money is left in the accounts. She won't share records. She has her own full time job, but I think she could probably claim she has helped with some administrative stuff here and there for his business. My other siblings don't understand what's going on and I think they'll defer to her since she's oldest and has the power/money.

I'm going to the probate court Monday morning, what do I say? Does she really get to keep everything?

He had 2 cars, another house (not very nice), and probably hundreds of thousands between the bank accounts, maybe even low million including the business.

Edit: more details, if its helpful. Sister was raised with us, just not formally adopted by him. When our mom died, we split everything evenly. I think the other house is just in his name. Its not worth a ton, bad condition, but my sister actually lives in it right now (he let her). I think the bank accounts had her on there because it was convenient. He wasn't very mobile or tech savvy in the last couple of years. He would drive over to bank branches or call -- never mobile banking. She helped with some of that type of stuff. I suspect she was added to some of the account fairly recently, as his health went down, like last year to 6 months. I am almost certain no one has gone to probate so far. Honestly, I don't think they'd want to. I know she has moved some money between accounts for the funeral expenses, she initially claimed there wasn't enough, as she is cheap and wanted a smaller funeral. Then magically found enough when all of the family was saying we need a real funeral.