How do i write a letter asking my son and his wife to move out of my home due to lack of respect and unacceptable behaviour from my son's wife

If writing a letter is the most comfortable way for you to confront them, that is understandable.
Just be honest when you write and make sure that you let them know how you and your wife feel. Remind them that they are living in YOUR home and that they need to respect that and if they can't then it is time for them to move on.
Do not put up with them making you feel unwelcome in your own home, you can not leave, they should if they are not going to respect you. Tell them that they have 30 days to find a place to live and if they do not, you will legally evict them. Once the 30 days is up and they have not left, you can get a legal eviction, it sometimes has to be a 60 day notice to evict, but it is worth it if you are unhappy. At least it will come to an end and you wil not feel like a prisoner in your own home. Maybe they will wake-up and start respecting you more.
Sometimes we LET people treat us badly because of how we react to them. If they disrespect you and you do nothing, or say nothing, they will keep doing it, without a second thought so now you need to let them know that from this day on, they will respect you, your home, your wife, EVERYTHING YOU or they will not be welcome any longer!
Good luck to you and please let us know if we can help with anything else! Keep us posted!
-Lissa