Every situation is different, Ginny so do not assume that your MiL will become a nagging pest and take over running your life when she is widowed - and who knows, it may be frail FiL who is left to your tender mercies.
The best thing is to mentally put yourself into the same position as the person you fear is going to become a burden and an irritation. Just what is their life like, or what will it be like? What would you need in the same circumstances?
Imagine what it is like to be them. Has their world been turned upside down recently, and the ground shifted beneath their feet by losing one half of their life with the death of a long-loved partner?
Are they having difficulty planning their own life and in need of support and companionship while they reconstruct? I know that it is not easy, and I am only 75 and in good health with a loving family.
Do they need a support to help them through problems or a whipping-boy for everything that is wrong in their lives?
Do they have a full life, with a lot of stimulation and contact with other people, as well as you?
What is their health like?