Should we call there baby and our baby siblings or cousins?

If you haven't seen my post in the pregnancy group, my husband's best friend is pregnant with our baby! Conceived the first time! We are so happy. And how she told was was saying "YOU GUYS ARE PREGNANT!" She's our surrogate because my husband donated his sperm to her and her husband who is trans so that they could have their now 10 year old son. One thing we are wondering is this. Both my body couldn't hold a pregnancy and my eggs were bad so she signed papers with our lawyers donating her egg to us. So our baby and their son would be biological full blood siblings... We were wondering... Do we call them siblings or cousins. She says we can just call them family and when our baby is old enough to understand our process they can decide what they want to call each other. Her husband said at the end of the day or doesn't matter because we are all one big happy family which is true. I was just wondering your opinions?

Just wanna add! Based on last period her due date is March 18th!