‘I slept with my sister’s husband and feel awful’

Dear Rhona, I don’t want to give myself away here so am going to be a bit vague with details.   I’ve fallen deeply in love with my sister’s husband and don’t know what to do.  They’ve been having difficulties in their marriage for a while. He feels that she prioritises their kids over him and says they don’t have sex anymore.  She orders him around a lot in public and kind of hisses at him...