Haha, it is not appropriate for parents to come forward in such a matter. As a younger sister, you have unique advantages. Maybe your coming forward will make them reconcile. There will be various frictions in marriage life. Quarrels are normal. Quarrels will also make each other understand each other better and pave the way for future life. At this time, others are needed to adjust. It is recommended that you pull them together and listen to what is going on. Maybe, without you having to speak, they will realize their problems and reconcile.
Of course, you can also give them a small gift to let them know that many people care about them. I recommend a few small gifts, real dolls, made into the appearance of your sister and brother-in-law, and placed in front of them, maybe there is really no need to say anything.
Crystal music box, good music makes them recall the past, and that feeling of happiness will return to their lives. There are many other good small gifts, you can choose and see, it will be helpful to you.