I think that they need three basic things to feel welcomed and loved--time, place and attention.
Time - Set aside some of your normal activities while they are visiting. Take some time off of your work, if possible. Don't talk about the things you are usually doing. If guests feel as though they are keeping you from the activities you would rather be doing, they will feel as though they are "in the way".
Place - Make sure they have a comfortable place to sleep, put their clothes, toiletries, etc. If you have a guest room and bath, this will be easy. If you have a smaller home, you will need to temporarily relocate some of your things to make a place for them. By taking the time and trouble to make a place for them, you will make them feel welcome.
Attention - Plan to prepare some of their favorite foods or go out to their favorite restaurant. Before their visit, ask them if there is anything special they would like to do or see. If they don't give you any suggestions, do a little research on your own. Maybe your mother-in-law loves flowers, and you learn there will be a home and garden show in your area during their visit. This would be a great activity for you to do together and really show that you care.