They will sleep better with a minimum of fuss. By being consistent you are helping your children to behave consistently. One of the most misunderstood words in the english language. Learn to use it particularly with children and other people who are making unreasonable demands on your time. There is no need to be nasty, but it does not hurt to be firm.
Many people try to get out of family obligations and the like with excuses.
All that achieves are raised stress levels, and usually a whole stack of white lies. People will respect your right to your own decisions and appreciate your help or visits more when they do happen. And finally TIME OUT for yourself, your partner and yourself, and your children. Time out is a very effective way of dealing with children who persist in being demanding, destructive or who are prone to tantrums. They can be left alone for as many minutes as they are years old, and providing you do not give in to more tantrums, screaming or tears, your child will learn through your consistency that their bad behaviour will not be accepted, and they will go on to other things.