I think you’re right that they didn’t share the type of closeness that William and Harry shared.
Meghan and her sister Samantha were 17 years apart, with different mothers, and obviously at different life stages.
Still, there are plenty of pictures out there of Meghan and Samantha together, from when Meghan was a baby, through her teenage years, that make Meghan’s claims of no relationship questionable. To what degree they were close, none of us know. But to say they were never close is contrary to what I have read from her family.
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I know that even today, in the age where everyone has a camera in their hands at every moment, I sometimes forget to take pictures. It happened to me just last week.

In these days (above), you had to actually work at taking pictures, with an actual camera. Sometimes there was no film available, the battery was dead, you had no flash bulbs, etc. So the fact that there are this many photos of them throughout Meghan’s life stages tells me that there were likely plenty of more times that they were together, too, that didn’t happen to get photographed.
Thomas Markle loved his children and grandhildren, and there are many pictures of them all together, besides these.
I understand, having older siblings myself, that, in some ways, Meghan felt like an only child. But she wasn’t. And she did have a relationship with her siblings, her father, nieces and nephews, and uncles and aunts, etc. It was hurtful to them when she acted like she didn’t have family, once telling Harry that his family was ”the family she never had”.