For two months, Edgar nurses Catherine, and though she improves somewhat, she never fully recovers her health. During that time Catherine does learn, however, that she is pregnant. Edgar hopes the child is male, so that the baby, rather than Isabella and Heathcliff, will inherit Thrushcross Grange.
It's not the depth of Edgar's love that's less than Heathcliff's, it is its nature—his love is nurturing and passive, as opposed to wild and unruly. Note also how Edgar has again taken on the feminine role of nurse.
Six weeks after she ran away with and married Heathcliff, Isabella writes to Edgar, begging for forgiveness. Edgar doesn't answer the letter.
Isabella, who is the most feminine woman in the book, is also the worst treated..
Isabella next writes to Nelly. She says that she is living at Wuthering Heights and that her experience has been awful. Heathcliff has told her that since he can't get to Edgar to punish him for Catherine's illness, he'll take it out on Isabella instead. Hindley, Hareton, and Joseph treat her just as badly. Isabella also writes that Hindley is completely unhinged and plans to kill Heathcliff and take his money. Isabella says that she has made a mistake, but knows it's too late to fix it. She begs Nelly to come visit her at Wuthering Heights.
Heathcliff's love for Catherine is all-powerful, and he isn't altogether incorrect to blame Edgar for her sickness. He wants revenge against Edgar, true, but he lets Catherine make her own decisions. In contrast, Edgar tried to "tame" Catherine, to make her choose a side rather than be herself, and this led to her sickness. Also note that Hindley now wants revenge on Heathcliff—revenge leads to revenge.