I can’t read Telegraph stuff (it’s all paywalled where I am). But re the curtsying and other formalities within the family, it seems like Harry wasn’t the greatest resource:
On introducing Meghan to the Queen (Spare, Sec 3, Chapter 15): '... as we neared Royal Lodge I got word on my phone. Granny was there. She’d popped in. On her way from church back to the castle. Meg said: "Fun! I love grandmas." I asked if she knew how to curtsy. She said she thought so. But she also couldn’t tell if I was serious. "You’re about to meet the Queen.” "I know, but it’s your grandma." "But she’s the Queen.”'
On introducing Meghan to William (Sec 3 Chapter 16): 'I introduced Meg, who leaned in and gave him a hug, which completely freaked him out. He recoiled. Willy didn’t hug many strangers. Whereas Meg hugged most strangers. The moment was a classic collision of cultures, like flashlight-torch, which felt to me both funny and charming. Later, however, looking back, I wondered if it was more than that. Maybe Willy expected Meg to curtsy? It would’ve been protocol when meeting a member of the Royal Family for the first time, but she didn’t know, and I didn’t tell her. When meeting my grandmother, I’d made it clear—this is the Queen. But when meeting my brother, it was just Willy, who loved Suits.'
On introducing Meghan to Charles and Camilla (Sec 3, Chapter 17): 'There, at last, on the landing above us, stood Pa. Beside him stood Camilla. Meg and I had rehearsed this moment several times. "For Pa, curtsy. Say, Your Royal Highness, or Sir. Maybe a kiss on each cheek if he leans in, otherwise a handshake. For Camilla, no curtsy. Not necessary. Just a quick kiss or handshake." "No curtsy? You sure?" I didn’t think it appropriate.'
To someone from a society without royals, aristos, nobles, etc., it must be a confusing situation: meeting members of The Royal Family in a private social setting AND as your partner's grandmother, brother, dad, stepmother, etc. Even if Meghan knew the formal protocols, it's not clear that there's agreement even within the family when and how strictly they should be followed.
Royals aside, I think psychologically it's easy to acquiesce to doing things that maybe seem a bit formal or old-fashioned in order to humour a partner's elderly grandparent or 60-something parent. It's likely a bit more of a surprise/adjustment if it's their 30-something sibling and family.