Complex sentence: His sister was the social reformer Eglantyne Louisa Jebb, founder of the Home Arts and Industries Association; his niece, Eglantyne's daughter Eglantyne Jebb, co-founded the Save the Children Fund and wrote the Declaration of the Rights of the Child.
Split into simpler sentences: His sister was the social reformer Eglantyne Louisa Jebb, founder of the Home Arts and Industries Association. His niece by her, Eglantyne Jebb, co-founded the Save the Children Fund and wrote the Declaration of the Rights of the Child.