Distributing one's time equally between one's wives

Can you please let me know what is the ruling regarding spending time with each wife. My sister's husband has recently got married to another woman after being in a relationship for many years. He states that having more than wife is a Sunnah but he is totally disregarding the feelings of his first wife and spends most of his time with his new wife. Can you please clarify how he should spend time with each wife. Also I find it hard to accept his reasoning that it is Sunnah. I feel that the word Sunnah is just covering up his wrong doings. Wrong is wrong. Please do not misunderstand me I understand fully the purpose of more than one wife and it being Sunnah. I have no objection to that at all. Loooking forward to your reply.