msavazzi I think this is the same as this one: https://community.tonfotos.com/d/732-relationship-to-improve
Currently program only adds automatically what is 100% certain. However, there could be siblings from different marriages and therefore program just does not know which family tie is certain and which is not, and adds only when it is 100% sure.
However, if instead of adding brothers and sisters you would add children to same parent, those children would become brothers and sisters automatically, as there is no doubt for the program. Since they have save partent, this means they are siblings.
So if you just change the order, and start from adding chilren/parents, this will work. But when you starting from sisters and brothers, there is still room for misinterpretation that is why it does not work.