How to Correctly Refer to Siblings Who Are Twins

Plural of blind (verb) is 'blind'. For example,

Thick shrubs blind my downstairs windows

However for something singular you would say 'blinds'. For example,

Detecting glaucoma before it blinds you

But in case of 'Blind' (noun) 'that we use in our homes' you will need to use 'blinds' for the plural, for example:

There are many different kinds of window blinds

There are two cases for (my) son:

1. Church context:
   One reason for celibacy in the catholic church is, that the priest should not be "distracted" by his own family, but consider his congregation as his family with him in a paternal role. Hence addressing him as "father XYZ". So it's perfectly acceptable to call his parishioners "my son" or "my daughter". This father/child addressing works in many different languages because it expresses an understanding of their relationship and tasks. The "my" is fine here.
2. Seniority, often protective:
   Usually used without the "my", the address "son" expresses the - temporal, usually just for the moment - assumption of a father-like role (or mentor) by the speaker to the addressee. Think of "it takes a village to raise a child" (from a different origin, but same thought). It always carries an undertone of seniority, often reflected in the age difference. Normally, the speaker would use "son" in a well-meaning way, even when admonishing. This seems only to work for boys, though, I have never heard the phrase "daughter" in a similar context. "Kid" might perhaps work for girls.

EDIT:
just for the sake of completeness, as requested ^_^:

3. Expressing the social or biological relationship:
   Father or mother talking to their son. But I'm quite sure that's not what OP was asking...