Complex sentence: He felt that Liu Ying, his heir apparent who was born to the empress, was too weak to be a ruler, so he had the intention of deposing Liu Ying and replacing him with another son, Liu Ruyi, who born to Concubine Qi.
Split into simpler sentences: He felt that Liu Ying, his heir apparent (born to the empress), was too weak to be a ruler. Thus, he had the intention of replacing Liu Ying with another son, Liu Ruyi, who was born to Concubine Qi.