First of all - do not panic. As long as you have 30 minutes, you can clean house fast. It won't be perfect, but it will be good enough so that you do not need to be embarrassed. And besides, you shouldn't feel too bad. If your in-laws are dropping in on you unexpectedly or giving you very short notice that they are coming, they should be prepared for anything. If they want your house clean when they come, they should give you advance warning.
The first thing to do is work on the smell. If the house doesn't smell fresh due to a pet or just overall lack of cleanliness, that needs to be addressed first. The first thing anyone is going to notice when they walk inside your home is how it smells. If you have any type of air freshener on hand, use it and use it liberally in your living room, kitchen, bathroom or any other area that you think the guests may enter. Candles are also an excellent idea. Light a few scented candles in the rooms where you expect your guests to go. Doing this early before they arrive will give the candles time to do their work and mask any other unpleasant scent that may be lingering.
After you've taken care of the smell, check all the surfaces. Clear clutter off every surface that your guests might see. Coffee tables, end tables, kitchen counters, dining room tables and bathroom counters need to be cleared off. If you don't have anywhere to put the stuff away, just stuff it into a cabinet or drawer for now and worry about where it actually goes later.
The floor needs to be tackled next. Pick up and put away anything that is in the floor that doesn't belong there. A quick way to do this is to just throw it into a bedroom and close the door. Run the vacuum cleaner over the carpet and mop or wipe down wood floors, linoleum or tile. If you don't have time to do the entire floor, just hit the spots that need the most attention.
Last, but not least, look at yourself and your family members. Anyone who isn't dressed and decent needs to get that way after you clean house fast. Don't worry too much about hair and makeup, just throw some clothes on so that you look presentable. If you're a female with long hair that's very messy, just put it up in a bun or ponytail and forget it. You've done your best.
If the guests that are coming are people you are very comfortable with who have been in your home a number of times, obviously you don't even need to go to this much trouble. However, for most of us this stuff tends to happen the opposite of how it should: the people you don't clean for drop in when the house is clean, and the people we always clean for show up when the house is dirty. Life is funny like that. If all goes well, you can be at the front door, waiting for them with a glass of iced tea and a smile!