"I don't expect anyone to tell my child how to behave"
It sounds as though your SIL has had to hear what her DD should be doing, at what age.
Developmental Delay isn't strictly speaking SN, it can right itself. Intervention, unless it involves severe speech delay isn't offered at this age and isn't needed.
Boarder line DD means nothing, tbh, toddlers vary.
Tbh, if you had of compared your DS to my middle DD, he would seem "delayed" she could if asked for a biscuit in a complete sentence at his age.
I agree that you don't like her, it looks as though she has had a rough time in the family, yet she cooks and hosts for you all.
Stop looking for something that isn't there. You have all judged her in the past, she may have called your child boisterous, why shouldn't she if you are judging and talking about her, as though you all know best.
The child didn't need an assessment at that age, just a hearing test.
I speak as a parent of two children with SN, one with moderate DD (which at 16 has righted itself) and Moderate LD's.