You are one incredible lady to do this!
I’ve skimmed over the other members responses ( no offense folks, I’m suppose to be mowing the lawn right now) and Squirrel came up with a really good one, don’t offically adopt them because they may qualify for college loans and grants in the future. Damn good piece of advice. When things settle down more, you can do some research into this an give the kids some kind of safety net.
Swiddles had a couple of excellent points. Short, coherent and to the point, none of which this post is 
There will be times it will be hard and very scary because it is new to you.** Remember two things: Patience and education. **
An educated person ( and parent) is less likely to spaz out over the little things ( you will discover for yourself what the little things are and what the big things are. This falls under the chapter of: Pick Your Battles) Read read read, ask a lot of questions to parents you respect and admire.
Since there isn’t a huge age gap between you and these boys, if you think of yourself more as a mentor than a parent, it may help you out. After all, it’s not the age that is so varying, it is the experience.
Also remember that there are things that are really big issues to them that you, me and the fence post know are just small potatoes in the long winding bumpy road of life. But to them, they are HUGE and try not to blow them completely off, but don’t allow yourself to become completely sucked into a hormonal emotional hurricane that kids can just pull you into. They have to learn to weather somestorms in life. Try to be their port that they can come to anytime things get rocky. ( Wow, look at all those metaphors!)
These kids will be going through alot of grief. Some lash out, others withdraw. If they lash out, they need to rechannel their energy in a positive way. Painting, writing, yard work, I dunno. We just were ordered to mow the yard no matter what our crappy feelings were. And in reality, mowing the grass is quite therapeutic.
If they withdraw, you need to gently pry them out of their shell. some kind of cancer survivor class/group thing will help you out. (They didn’t have that kind of thing when I lost my dad - or maybe they did, but we didn’t know about it.)
Do expect the holiday seasons mood to be quite terrible for a couple of years. Maybe after a few Xmas’s, plan a trip or a special holiday event that can help them overcome their grief.
I am a huge proponent of planting a tree in honor of someone after they pass on. Doing something like this will help keep their spirit alive in a positive way.
Having your mother in law write down ( or you write down for her) the things she would like for her children ( good manners, education, respect others, the rules or whatever) may help them when she is finally gone.
This is a bit morbid and odd, but that is pretty much who I am as my life has been defined by death and morbidity,but I would have given anything for such a thing. If your mother in law is up to it, have her write letters for her children to be given to them when the graduate high school, college, get married, have children. You know, the milestones that she will be there in spirit for. Then you can give it to them on those days when the sun in setting low.
They don’t have to be long winded or anything, just vagueness entertwined with emotion. " My dearest bobby, I wish I could be there today to see you graduate from high school. I know you worked very hard and have overcome alot in the past few years to reach this milestone in your life. I know it hasn’t been easy, me dying on you was not planned and I know Brunetter and your brother have done the best they can being put into a situation that they could never have possibly imagined…"
As for parenting advice, I have only two adages that I can share with you ( for free!)
1> Never miss an opportunity to take a pee.
2. I’ve learned more from a bad parent than I have from a good parent.
Remember, if you ever have any questions, concerns or problems, you know that the folks here always have open arms and are willing to give you their two bits to help you weather the storm.