Might have been a mammary tumor, but those aren't usually malignant, they just keep growing and growing not causing any harm. Maybe the other 2 gave her something that they carry and don't show signs, but she caught it and showed signs of whatever it was. Maybe Mycoplasmosis.

Breeder females really should be alone if possible, and introducing those 2 new ones was a bad idea. It probably stressed her out and they either ate the babies, or she did out of fear. The reason it's best to keep them alone is that other moms may steal the babies, and then the babies would die because the stealing mother wouldn't be able to provide enough milk for all of them. Or the other mothers would kill the babies, which sounds like what happened in your case.

If it's possible, and you have another cage, I would probably separate the remaining 2 mice and just cater the male to them That would most likely be your best option and there wouln't be as much of a risk of stolen/eaten babies. Hope that helped a little and I'm sorry for your loss. I know your sandboa was looking forward to those babies