Its commonly known as "Low muscle tone" there is a thread down the list you can look at.
Although there are a lot of children around with LMT, most people have never heard of it. There are very varying degrees of LMT too. My daughter has a mild form of it and she didnt sit until 10 months, crawl until 12 months and walked at 18 months, she still walks like a much younger child even though she is almost 3 yrs old.
Sounds like your cousin is getting the help she needs though with physio and speech therapy, some problems may not yet be apparent. I may be able to find some links for you to look at if you want.
Dont worry too much, Hypotonia is a lot more common than you'd think. You are great to care so much for your little cousin!
hope this helps
DH Phil 42
DD Naomi Helen (dob 16/3/01)
Bumblebee EDD 8/04/04