Hi Dina,
The situation is slightly reverse for me, i’m the eldest and my brother has never EVER listened to me before 😉 The only thing i can offer is maybe consider doing online sales? Quite a few businesses here have online cupcake shops and just bake to order. OR you could set up an intermediary/ third party between the cafe/ bakery and your sister.That way she could just focus on baking. One of my bosses had social anxiety and so she would just hire casuals to work for the front counter while we were in the back baking and cooking. I also know another person whom suffers from severe insomnia and social phobia. She became the third party by buying wholesale toys from japan and selling them in stores. There are options, although your sister may have to meet a couple people, if she has support, then she may be more likely to give it a shot. I always think of a building and its scaffolding. It wasn’t built over night, but the support the scaffolding gives to the building is incredible. Once the building is able to stand without support, the scaffolding comes down, gradually. Maybe you or someone else your sister is close to could support her, professionally (you are already supporting her now) until she is ready to stand on her own.
It was just a thought. Best of luck.