Sunil Shera or what ever name you have.You Sangh Pracharak always say half stuff every time. The daughter-in-Law you are talking about was not his daughter-in-law at all.Infact Prophet never had a son.His wife khadija had a slave boy who was an orphan our prophet adopted him as his son.Infact it was he who has set this marriage of this adopted son with lady you are talking about.Since lady belongs to a very rich family.Family opposed this marriage.but it was proposed by Prophet they agree.Prophet want to have strong bond with this tribe of that lady.But differences betwn his adopted son and his wife were so deepen after the marriage that his adopted son gave Divorce to his wife.In order to have strong bond with the tribe he married that lady who have been divorced not for any lust.The son you are talking about is not his blood relative nor he had much of emotional relationship with the profit. But detail answer is not possible here I am just giving you short answer. Better think openly not finding fault in others religions.I do respect your fate so I don't wanna talk about it.