OP, I do understand why your mother is concerned with whatever it is she is hearing from your sister. It does seem as if your BIL is taking advantage of her by the way he spends money coupled with being tone deaf to her needs (like booking a tour when she wants to stay put).
But, your sister is not a victim. She knew that he was going to medical school before they were married. She knew that he made these decisions that would greatly change what their life together would be and she still married him.
Now, people do stupid things for love all the time. They ignore red flags and they give up their own dreams and aspirations in favor of what someone else wants. It's unfortunate but it happens all the time.
Given that he is using "his own money" to pay for school while using money that your sister earns for every thing else it does seem to me that he is using her. But, she is allowing herself to be used.
There is a certain power in playing the victim and that what it seems your sis is doing in complaining to your mom all the time.
You say: What I do have a problem with is that he proposed shortly after, knowing that he had no job, and wouldn't have one for 5 years.
Your sister accepted the proposal! He didn't force her into it. I think that needs to be the basis of any conversation that you have with you mom. Sis is an adult. She made the decisions that she did knowing what the deal would be.
And since you are paying attention to all this, as some have said, realize that if you someday call mom to complain about the same sort of thing, you know what she will be liked. But, more than that, seeing how your sister is being treated and being unhappy about it, make sure that for yourself you don't end up in the same position.
Because while you think it couldn't happen, love does strange things to people and logic flies out the window!
Also, you may want to tell your mother that by taking her daughter's side against your BIL that it leaves your daughter in the position of actually defending him when that may not be what she really wants to do. In other words, your mother's constant criticism of the situation could prolong it when it would otherwise fall on its own. The trick is being supportive without influencing or otherwise playing a part of the drama of their relationship.
When sister calls to complain that he booked the tour, mom should say, "well, you (your sis) didn't have to go. You have your own money, you could have stayed where you wanted to stay."
It really can be as simple as that. When your sister is made to accept responsibility for her own decisions and their consequences, maybe she will be a little more careful about making them and standing up for what she wants instead of going along and then calling mom to complain.