It's hard to know the best way to entertain. If you feel you must make your husband's family's ethnic specialties, and you don't feel sufficiently schooled, that can add stress. One way to do it (for the future, maybe not for the next event) is to schedule a "kitchen tutorial" with the 2 best cooks in his family. Let them show you their tricks and techniques, show them a genuine interest in learning more, etc.
However, the other way to do it is to make it YOUR event and do what you feel comfortable doing. A BBQ is pretty standard stuff. What I do is make a reverse schedule: I start with what time we are going to eat, and I back up from there, writing down what happens at what time. If "serving time" is 4 PM, then I have a list for 3:45, 3:30, 3:00, etc. I make as much ahead of time as I can, and I have a spare fridge in the basement to put things. If you don't have that, see if you can implore a neighbor to help you with everything from extra ice to a shelf or two in their fridge.
Go for simple but flavorful recipes. I have a pretty firm rule about no dish taking more than 5 ingredients, and most being "make ahead". You can also let the stores help you out by buying prepared foods and just putting them in your own dishes. For example, use the store's cut-up veggies or fruit, and add a tasty 'designer' dip and your own platters. Make things interesting by putting the dip in a wine glass or margarita glass, for example.
I also get all my platters and bowls, pitchers and baskets, out ahead of time, including serving pieces, and I put a sticky note on each one saying what goes in it. Everything occupies the dining room table with a label. Then when it's time to prepare or serve, that work is done.
BBQs are usually burgers, dogs, chicken (with a sauce or marinade), and side dishes that don't contain mayonnaise (which can't sit outside for a long time). You can keep things cold by putting dishes in a larger dish filled with ice. A lot of times, people use the large foil pans as are sold for chafing dishes, add a layer of store-bought ice, and put the 2 smaller foil pans on top. Of course, the chafing dishes themselves are good for keeping grilled food hot. Ahead of time, make a platter of lettuce leaves, sliced tomatoes, and onion rings (regular, Vidalia or red) and just put a fork on it for serving. Make smaller dishes of common condiments (ketchup, mustard, relish, pickles) if you don't want to serve in the original bottles.
Do you need recipes of simple side dishes? I have a lot, and you can go to places like Rachael Ray for pretty simple recipes for a crowd.
Starting now, you can take a set of plastic cutlery and roll up 1 knife, 1 spoon and 1 fork in a napkin, then tie with a little piece of cheap curling ribbon, then stack these in any cute container - a basket, a decorative (clean) flower pot, etc. It speeds up the buffet line and it looks cute, like you fussed a lot. But you can do it while you're watching TV.
For drinks, buy bottled single-serve iced tea, lemonade, etc. and put them in a decorative tub (on sale at every place that has picnic and BBQ gear), then dump a bag of ice on top. If you are serving beer, put it in a cooler and tie a bottle opener to the cooler handle with a length of string or ribbon. Put a few bins out to collect recyclables as well as trash.
If everything is made ahead except the grilled meats, then you employ your husband and your children (if they are old enough) to help get things ready. If you have a list of what goes out at what time, you can just follow your cheat sheet instead of stressing about what you forgot.
At some point, you really have to get yourself to the mental place that you are who you are, and your worth is not measured by someone else's estimation of your cooking ability. If they don't like you, they don't have to come back! Your husband married you even though you are not from his ethnic group, so take comfort and pride in that. It's also okay to share YOUR culture with them, with pride and yet with admiration of where they come from.