I don’t know too much about working with The Morrigan as she hasn’t made an appearance in my life or work, but I do have links I can share with you to share with your sister!
Here are some posts from the forum:
– The Morrigan: Celtic Goddess 
– Has anyone else learned about the Morrigan?
– A prayer or chant to Morrigan
Here are some blog posts and videos from trusted sources:
– https://loraobrien.ie/morrigan/
– Is the Morrigan Calling You? What are the Signs? - YouTube
– Exploring Celtic Mythology: The Morrigan - YouTube
– The Morrigan Resources Guide 2022 with Lora O'Brien of the Irish Pagan School - YouTube
– The Morrigan - Sources in Irish Mythology - Lora O'Brien - Irish Pagan School - YouTube
– How Do You Recognize Bad Irish Paganism or Celtic Pagan Sources? | The Irish Pagan School - YouTube (throwing this one in here because there’s a lot of really terrible information about Celtic paganism and deities online - it’s always good to know how to recognize bad information!)
Here are some books from trusted sources:
– https://www.amazon.com/Pagan-Portals-Morrigan-Meeting-Queens/dp/1782798331
– https://www.amazon.com/Morrigan-Celtic-Goddess-Magick-Might-ebook/dp/B07PLP8NV2
I know there are a few people here in the forum that work with or worship The Morrigan in one way or another, so hopefully they can chime in and share personal experiences and more information with you!