Am I being unreasonable?

So basically my so has stormed off after arguing and I left feeling I am being unreasonable. Rewind: my so and i had a very good relationship with his sister she helped us out with food and bills occasionally. We got into a disagreement about our son and she cut ties with us threatening to call social services.(our son is autistic and attends a special needs school).she works with disabled children and thinks we are not looking after our son the way she thinks we should be doing. She refused to talk to my so and tried to stop his visiting their mom (She lives with her mom and fiance) but now she's arranged for her best friend ( who happens to be my so's ex girlfriend) to come down for the weekend she's being all nice again offering to do a food shop and asking if we need money, just because I found out she wants my so to babysit his ex's kid at his moms house while they go out. I told him I don't want him to do it because I'm angry at her for threatening social services on us and she's been a complete cow to us the last couple of weeks. He's said he's doing it anyway saying he can do what he wants. Am I being unreasonable? I want my so to have the relationship with his sister again I'm not stopping that, it just feels like the minute she needs something from him she's all nice again. Please no hate comments I just don't know what to do for the best.