I'm sorry for the delay. I had an appointment and just returned.
The most common cause for a dog to suddenly break house training is parasites or a protozoan infection. Parasite are often contracted when a new dog is added to a household. Let me give you some information on parasites and protozoan infections. You can read about these here:
Worms and protozoan infection make it difficult for a dog to control their bowels and thus Rosie might find it difficult to hold her bowels if that is the case. You might take a stool sample into the vet to have it check. Be sure they look for protozoan infections.
What might help until you have the results back or can retrain her would be to either move her big meal to an earlier time or a later time so she does not have to eliminate in the middle of the night. What many people do is split the evening meal into two and give one at 5pm and another at bedtime. This keeps the dog's stomach full, which allows them to sleep better, avoids excess bile which might wake them and hopefully delays the need to defecate until morning.
I am a firm believer in crate training so definitely crate her but if it is due to parasites, she may soil her bed. I would not let the new dog sleep on the bed if she is not there as well. It would upset her place in the household.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.