OP, now you know that when/if you get married, your mum will probably discuss your private business with your sister and be all up in your marriage. So, similar to the "snooping" threads advice, don't say anything you wouldn't want up on a billboard for all to see.
Was this a marriage to a man outside your culture or someone from an income bracket that might as well be on Pluto compared to yours? It's possible that you just plain don't know him or his background very well (if you don't see him much). OTOH, you may just not like him, and that's OK. But, your sister chose him, and chose all the other situations, too. Be supportive, but stay totally out of the advice department. If she feels the need to change, she will talk to her husband.
If you were asking our opinions on gender roles and breadwinners- there were many, many couples who permanently or temporarily depended on a wife's income, even back in the "good old days". Providing has nothing to do with the qualities that make a good husband. You want to avoid coming across as judgemental, as I'd be willing to bet on a board this large, there are couples with any kind of arrangement for short or long-term.