My husband and I had to help both our mom's out financially during our marriage. Unless you both make a lot of money and have very secure jobs, these are not times to be wasteful or unaware of where every dollar is going. I advise that before you confront your husband about the sum you are giving to his mom, the first thing you do is sit down together and look over past tax returns. Look at your monthly expenses and annual expenses. Make sure you factor in savings and emergency funds, college accounts and retirement, auto maintenance and replacement and household repairs and maintenance, health-care premiums and annual deductible, life insurance, other insurance, annual vacation, clothes, entertainment, cell phones, cable, EVERYTHING. Add into that what you give your MIL and other charitable giving, allowances for the kids. Total these expenses and divide by 12. Are you able to cover these expenses monthly? If so, how much is left over each month?If not, begin evaluating every line-item in the budget. Some are non-negotiable but others can be adjusted.
If the money you are giving his mom is not causing a problem, just leave it alone. After all, she made the sacrifices most mothers make to raise your husband and take care of him and it surely blesses him to see her able to have a season to spend money for herself. However, if your own household budget needs to be reigned in, you have in impersonal opportunity to say, we need to establish a more realistic household budget for our family and should also help your mom learn to budget her money as well so we can continue assisting her.
There are many chirches who have financial counselors, classes ( free) and accountibity partners. I recommend them for every one, regardless of their financial situation. One is offered through Crown Financial Ministires and the other is Dave Ramsey's Financial Freedom University. The former would be great for your MIL and the latter ( or both) would be great for you and your husband. These are more than money management, but are great training to understand how money works and how to make it work for you. Your MIL would love it and meet new people.
Good luck!