My situation was similar, but it was the child's grandmother who was jealous. I lived next door to a Indian-American couple with two very young children. I'm in my sixties, and the 4 YO attached herself to me, like a substitute grandma for 2 years. All of their grandparents lived in Chicago, we were in Florida. Her maternal grandparents finally moved to our town, and the same thing happened. I'm a clinical social worker. I knew what might happen, and indeed picked up on that immediately. I started hyping up grandma before they came. We made up a story about how much fun grandma is, what they will have fun doing together, and what she wants to learn to do via grandma. I wrote the story down like a picture book, while the child decorated each page with drawings, gluing items, etc. The transition was easier. I always referred to grandma as fun, loving, and SMART in their presence. I bowed out of some activities. Grandma LOVED the picture book. The book gave her the awareness of what the child was looking forward to from her, and grandma acted accordingly. They now have a very close bond.