Difference between the sentences

I imagine it was on another thread where you asked a similar question.

No, I've done enough research for you. Just google "Use of Commas" and many, many websites will appear. Read them to find the answer to you question before you come here. They're very good sites. I used them when teaching ESL.

With no response from you, I have to conclude that I didn't receive the comma links.

If it's easier to ask native speakers here, then just continue as you have been doing. I suspect that, were you to read the links about commas, you'd still have questions that are more easily answered by simply asking a native speaker. But Mame does have a point; there are many sites which explain the use of commas – or attempt to. Here's one:

https://eslgrammar.org/comma-rules/

I believe a big difference. The comma on either side of Mary is to say that a deeper connection exists not about or for Mary. For the reader is the audience and audience is more than one. Thus, the writer is identifying with an intimate section of the audience. Thus, it is My sister, Mary, is clever.
Identified are three separate parties:

"MY" who is HIS sister?
"MARY" who is HIS mary?
"CLEVER" who is HIS Clever?

Your word usage is unfortunate or fortunate:
You selection, electable, is special.

I am speaking to a party outside of you or me whom I have already had extensive communication with and they happen to be very dear to me. I am telling them that electable is known as a term and applicable in a very rare but non-unknown way. Euphoric writing can be for me.
This is Mark's Law and that is the greatest law.

Thank you - I know the comma to be primarily used for that.

That is so truest.
https://gifdb.com/gif/playful-chibi-zero-two-oyrheyl2jkufcdta.html

My formatting is messed up. Are you saying that it could mean that the "Mary" presentation was to say that the one called Mary is neither clever or sister per he own of "MY"?

Comma's cause the reader to pause, they break the flow and weaken a bond between words, this allows different interpretations of the following text

Without the 1st comma, "My" has a sister called "Mary", it is unambiguous
With the 1st comma, I know "My" has a sister but the comma allows for "Mary" to be a different sister

Without the 2nd comma (with or without the 1st comma) we are told "Mary" is clever
With the 2nd comma, the pause allows "is clever" to be interpreted as a question

So yes, with both comma's present, Mary may or may not be "My's" sister or clever