What should someone do if they are being distracted from their wife by being attracted to/ thinking about their wife's sister. if sometimes they wish they had proposed to her instead of their wife because she is a better person and better looking than her. there's nothing they can do as they are married so they want to stop feeling this way and also want their wife to somehow start looking the best in their eyes

One of the things about life is that there are always temptations out there that shaitan lays for us. If you're unhappy in your relationship then it might not necessarily be your feelings for your sister in law, maybe you need to resolve with your wife by talking to her. If you talk to her lovingly and let her know that you don't want to loose her, and that she has to try to make you feel that in every way, be it physically or emotionally, then im sure she will do so. Be patient and try your best at the same time. Your efforts will surely pay.
All married women would like to secure their marriage and your wife is no exception.
At the same time, you have to completely refrain thinking of anyone else specially her sister. Try to avoid any kind of meetings or confrontations by excusing yourself and seeking refuge in Allah from shaitan. Don't let anything remind you about her if you wish to keep your marriage and not do anything forbidden.
In Islam, you cant marry two sisters at the same time so if you end up separating with one sister for the other, just imagine the repurcussion of not only breaking a marriage, but also breaking the hearts of two siblings and the family bond they share.
You really have to focus on your marriage. If you focus elsewhere the marriage can't work. All relationships go through ups and downs but if one tries his best and wants to keep his marriage especially for the sake of Allah, then he surely will get the result of his efforts as promised by Allah Himself.
Remember, this is probably just a phase that you might be going through but never give in to the false temptation. Consider it small compared to the great things in life that Allah has created us for, like the ultimate love we should have for Allah only and all his creatures for His sake, and attachment to the Holy Prophet and his pure progeny, how Allah wants perfection for us through knowledge and practice like patience and thankfulness, and how we should have great aims in life to achieve these perfections.
If you really can't figure out how to get your relationship with your wife back on track, talk with a therapist or better still, a learned and experienced person and and get some advice, but don't just let it linger because shaitan will take over step by step until he makes you do something drastic which you will always regret.