Meghan's half-sister, Samantha Markle, tells TMZ ... their dad is in the hospital recovering from a stroke, and she thinks his health is failing due to the tremendous amount of stress in his life from Meghan.

Samantha seems to be drawing a direct link between Thomas' stroke and Meghan ... she thinks it's a byproduct of Meghan's refusal to have a relationship with her pops.

As we reported ... Thomas was rushed to a hospital Monday night, being reduced to writing his symptoms down on a piece of paper because he couldn't use his voice.

Samantha says TM is having mobility issues on his left side, is unable to move the muscles in his face, can barely use his left hand and is learning to speak again with the help of a physical therapist.

It's the latest health problem for Thomas, who has said he suffered 2 heart attacks right before Meghan's 2018 wedding to Prince Harry.

Those heart attacks happened while Thomas was in a tense standoff with Meghan and the Royals over allegedly staged pics in the media of him getting ready for the nuptials ... with some folks accusing Thomas of faking a medical emergency for sympathy. Thomas ended up missing the wedding.

Samantha believes Thomas' heart attacks are also related to stress from Meghan ... and she says he would not be in the hospital now if Meghan was a loving daughter.

Thomas is going to be 78 in July, and Samantha says there will be a long recovery from his stroke ... and she's hoping Meghan wakes up to the possibility her father won't be alive for long and will make a change.

Samantha isn't holding her breath ... she says life's short and thinks if Meghan hasn't regretted the non-existent relationship with their dad by now, MM probably never will.

And, get this ... Samantha says Meghan "better stay the f*** away from a funeral and not use it as a photo opportunity" if Thomas dies soon.