Hamsters are solitary animals and must be caged by a mouse. No matter how good their feelings are, they also have territory. According to you, they just had a fight, and then mom killed that one and probably ate some.

The dead hamster must be taken out immediately, the cage must be cleaned carefully, the basin, kettle and wheel must be washed clean, and then disinfected with hot water. All gasket materials should be replaced.

I hope you don't keep hamsters in cages again.