It appears to me that you are assigning the Multiple Birth fact to only one of the twins and assigning the principal role to one and the sibling role to the other. If that is the case then I think the sentences need to be something like the following.
Principal : [Person:first] and [Sibling:first:surname] were [Desc].
In other words, the sentence for the principal is just as you have written it.
Sibling: [ThisPerson:First] and [Person:first:surname] were [Desc].
This is not the same as the way you have written it. The trick is that the [Sibling] variable is really intended to be used in the sentence for the Principal. It is then the [ThisPerson] variable that is intended to be used for the name of the person in the role sentence such as the [Sibling] role in this case.
I’m going to have to think about it a bit to understand how to make this work for triplets. It would work fine for triplets in the Principal sentence because [Sibling] would expand to to cover both of the other triplets. But getting it to work for the Sibling sentence is a bit trickier. I’m pretty sure it can be made to work, but I can’t remember just now how to do it.
RM7 has nothing to do with it.It works the same in all versions of RM from RM4 through RM10.
Instead of a shared fact I simple add “twin of Henry” on Mary’s birth fact description, and on Harry’s birth description “twin of Mary”. Searching for twins is easier that way using search criteria: birth - description - contains - twin.
If you don’t see the description field on the birth fact you will need to enable it. Click on the 3 dot menu on the People page and select Fact Type List. Highlight Birth and click Edit. Check the box “Use description field”. Then edit the Principle role sentence template to include < [Desc]>
I like Renee’s approach as compared to shared facts for this particular use case. It also should make handling triplets or quadruplets easier if you have any of them.