@hellodarkness I agree with your post entirely.
OP, it is unreasonable to expect weekly help from family, they are entitled to their own life and to be fair they both are doing a lot for others as it is.
In your case as your child is of high needs, it may actually be better for you to hire help, if you search well enough you may even be able to find someone who has experience with children similar to your little one. Furthermore, hired help will be consistent and you won't have to worry whether or not the person will be able to help this week or not as you will have a schedule.
Speak to your husband about this and begin the search for someone to come in twice a week for a couple of hours just to help you out and give you a break. There is nothing wrong with this.
Furthermore, don't resent your parent's or in-laws, it could simply also be that they would find it hard to manage with your child, but do not know how to tell you. However, once things are set in place and you get the help you need you can work on a better relationship with them. But for now, focus on getting yourself better - you are the priority.
Your HV has crossed a line and should not have made such a suggestion, it is not her place.
Also, when your husband is at home, ensure he understands how you are struggling so that he can take more of an active role in his child's life.
I wish you the best, please do look for the right help.