Hi there Switz and welcome. What a wonderful legacy and sorry to hear about the loss of your SIL. What a lovely lovely way to remember her and an interesting question.
First of all, is the bag a pale colour? The first thing I would do is buy some leather protector and rain proofer. Any good cobblers/ shoe and bag retailer can advise you on the best product and I imagine that if you emailed the main Ralph Lauren stores, they would be delighted to assist you by providing you with after care advice.They will also help you to replace any hardware on it as it gets worn too. If it came with its own dust bag make sure it is stored in there and keep it stuffed with paper to keep shape if it is unused. Let it dry out slowly if it gets wet and keep away from heat sources.
Secondly, keeping an eye on any bag is always a good idea. I do not let my bag out of my sight regardless of cost and certainly a high end bag can attract the wrong attention so always hold it securely when out and about. You can buy little bag hooks to attach it to a table underneath to avoid dirty floors/thieves who may try to snatch it if it is left unattended.
Thing is, a well loved bag is a thing of loveliness too. It will inevitably get a little scuffed but that is part of its charm and maybe you will want to hand it down one day to a DD. So I would never advise you to sell it even if you end up keeping it in a cupboard most of the time.
I just cart mine about TBH. They can get as little ragged but your SIL clearly wanted you to use this and perhaps had an inkling that you might not
feel confident about it.