Sensitive Post- Vanishing Twin?

I was wondering if anyone has had any experience with this?

I went in today for my first ultrasound.

Baby A: is measuring on track with a heartbeat of 147.

Baby B: we could only see a yolk sac and it was measuring smaller. She didn’t give me the exact measurements.

The doctor told me that we will keep a eye on it and I go back next week.

My question is with vanishing twins , how will I miscarry it or what will happen? I just don’t want it to effect my other baby also.