"By agreeing to extend the lease, and with Jack and Eugenie focusing on Portugal, they have a firm presence smack bang in the middle of Windsor court where it's all happening," the source told The Sun.
"They can now come and go any time they please. The coming year could be one of the most tumultuous in royal history and the Sussexes are making sure they have a foothold in Windsor."
Harry and Meghan - along with their son Archie, three, and daughter Lilibet - are set to return to the UK for the Queen’s Platinum Jubilee during the national service of Thanksgiving at St Paul’s Cathedral on Friday.
Meanwhile, Eugenie and Jackie, along with their 18-month-old son August, had reportedly decided to make the move to Portugal after Jack landed a plush new job with businessman Mike Meldrum, with his new role including sales, marketing and promotion for Maldrum’s private 300-house private resort.
The resort is said to boast 300 homes and an 18-hole golf course, a spa and an equestrian centre.
A source said: "Jack is now working for Mike Meldrum and they are splitting their time between Portugal and London. It’s a very exciting time for them.”
Meanwhile, despite alleged rifts between the royal family and Meghan and Harry in previous years, the pair are said to be delighted to return to Harry's homeland ahead of his grandmother's exciting milestone.
"The couple are nothing but excited about coming here because it is a moment that's very much about the Queen to them," Omid Scobie told Foreign Press Association journalists in London.
"This is about celebrating the Queen's life, her 70 years, not about celebrating the institution but very much someone that's important to them, and I think, for them, they're excited to be a part of it."