The connection between Irene and Lorraine goes beyond the mere fact that the two actresses are siblings.
The film also portrays Irene as being in her late-teens/early twenties in 1952, which would make her around Lorraine’s age in the 1971-set The Conjuring.
The Nun actually ends with the seminar from the first Conjuring in which footage is shown of The Warrens exorcising a farmer, who is revealed to be the character of Maurice “Frenchie” Theriault (Jonas Bloquet) from The Nun!