Oh tShis is so hard....
You will feel your way and make good decisions.
To share a little with you. Just a week after our first angel died I attended the birth of a friend. This friend 'needed' me to be with her. She was having a vbac and was heavily reliant on my presence. She offered to release me from my commitment to her birth but I thought a lot about it. I knew that she needed some energy that I could give. My situaion is different in that I have birthed live babies before.
I didn't find it hard to deal with. I understood and honoured the reality that my baby was gone but this baby was coming.

I would urge you to talk openly and frankly. Honour how you are feeling. You ARE the birthing Goddess and this is your time. Be very selfish with that time. It is necessary. You need to nurture yourself and your baby. You can't afford the energy to nurse the grief of your friend. However, your sister in law may feel she can cope. Feel your way. However I would urge you to prioritise yourself at this time.
Love your friend, empathise with your friend but honour the sanctity of your birth as well.