According to royals expert and author, Marlene Koenig, Middleton likely won't even be in the bridal party, due to the fact she "barely knows Meghan."
"For another, she is the future queen consort when William becomes king after his father, and would unlikely be an attending person to another royal bride," Koenig told Town & Country.
Koenig added that while Middleton could hold the title of matron of honor, as she is married, it is unlikely that she would do so as she did not even hold the title at her sister, Pippa Middleton's, wedding in May. There, the Duchess chose to stay in the background as to not draw attention away from her sister.
“Kate would upstage her sister,” veteran royals author Judy Wade told People before the wedding in 2016. “It’s a tricky situation for Pippa. She would want her sister by her side, as who else would she trust to make things go well? But if your sister is a future Queen, she is going to upstage the bride and draw attention. Kate would want to be in the background as much as possible.”
The same situation is likely at Harry and Markle's wedding. There's also the fact that the Duchess is set to give birth to her third child in April, so it's safe to assume she'll be busy with her new arrival.
However, Koenig speculates that the Duchess and Prince William's other children, Prince George and Princess Charlotte, will play a role in the wedding, as they did in Pippa's nuptials. In Britain, the bride walks down the aisle first, followed by her attendants or bridesmaids and page boys.
"It is very likely that Prince George and Princess Charlotte of Cambridge will be a page boy and a bridesmaid," Koenig said. "Anyone else is a guess."