Chinese Hamster Gender

[LadyRia]

Hi,
I'm having a difficult time figuring out their genders.
The brown one is named "Sushi" and the white one is named "Snowy". They are both from the same litter and I've had them for a while now in a 60gallon tank with lots of room . Recently, Snowy has been acting weird and has been taking fluff into his hut, hoarding food inside and around the hut and has been aggressive towards Sushi. I tried to get the best picture of Snowy but I'm starting to notice that nipples are coming to be visible, he doesn't like being held right now so that's the best picture I could get. Last night Sushi had it with Snowy that he just put his brother in place (assuming Snowy is a he") .
Please help! I also have a feeling that if Snowy is indeed a girl, that Snowy is actually pregnant.