V.F.
First I would spend the money and join HSLDA. Then don't tell your mil. It's none of her business anyway. Make sure that you find out what the law is for in your state. DO NOT listen to the school. Find out for yourself or from a local homeschool support group what the law says and what you need to follow. There are many things that the school will try to tell you or say that you need to do and it will not be what you have to do. Make sure you understand what the requirements are for special needs children. They will have different requirements if your child has been labeled by the school district. If you are in PA go to www.chaponline.com to find more useful information. They also have a great homeschool convention coming up in May. They usually have some speakers there specifically geared towards special needs. They will have a plethora of curriculum and lots of people to give you advice and tell you about their experiences.
Oh and one other thing NEVER EVER let DFS or CS into your home! Never unless they have a warrant. They are not allowed in if you don't want them there.