I dont really buy how the two shows are the exact same.
Sure there are some geeky characters and a blonde girl, but the similarities end there. I dont watch the show often, but the girl on BBT is not Sheldon's sister. She just lives across the hall. She always ruined his plans, where the girl is just kinda there and something nice to look at. I dont think she ruins any plans for crazy inventions or anything.
Plus who the hell are the other two guys from the dexters lab thing? Friends of dexter I assume, but they played much less of a role that the friends of Sheldon do. IMO, stupid claim of a ripoff.