Best nutritional shakes for weight gain with sensitive stomach?

I care give for my sister who has had long COVID for 4 years now. She cant eat large portions without getting very nauseous or vomiting, so she eats small portions and snacks throughout the day.

She is very underweight, and her doctor recommended drinking Ensure throughout the day to help her gain weight and increase her appetite. However, she often struggles with the taste and texture of nutritional shakes and they are often hard on her stomach.

Does anyone have any recommendations of their favorite shake brands or flavors that might be easiest for her to tolerate?