bei dem vater is totally different
Bei is used when you are staying at someone's place
ich habe bei meinem Onkel geschlafen , oder zum beispiel
gestern war ich bei meinem Freund ...USW
don't hesitate to ask if anything is not clear
Taking for instance the PONS page for bei, the first example is bei wem hast du die letzte Nacht verbracht? with the meaning with whom… I understand the meaning of bei jmdm that you pointed at, at someone's place, but I'm not sure it's the only one.
Dative is when the noun is after a preposition, so "das" changes to "dem", and "in the house" is translated as "in dem Haus" = "im Haus" (in+dem= im).
That's my opinion!
PD: I hope you excuse me for my English, I'm not a native speaker :$.