RaspberrySnowCone this may be way out of line and totally stupid suggestion but if I were in your boat I would send her a small bouquet or a box of chocs from Next, Interflora, Moon Pig etc and a thank you note to arrive before she sees social workers.
The card would simply say "Thank you so much for being our referee. We know this process can be hard for us all and nerve racking. We are just so excited to be at this stage and very grateful for your help as our referee."
Either that or I would simply warm social worker that your mil is very nervous and this may present as hostility etc.
Or both and ask social worker if she thinks a bouquet for referee is acceptable.
OR ask on here and see what others say!
If your mum would see through this at a mile then do not do it.
I am just thinking what would put her in he best mood just before she sees SW!
Try not to worry. If you explain to your social worker your mil thinks it is none of her business then that may explain rather than she is hostile to you adopting.
(Just out of interest - why did you ask her, was she more amicable before? If so say to social worker she has got more nervous etc as time has gone on!)