So weird. Sounds like she's not right in the head.
Think
SIL BIL DH You.
As in, there are two brothers in between you. Two men whose failure to commmunicate and assert themselves is causing you upset.
Today, have a stern word with your husband.
Tell him how how distressed his SIL's behaviour makes you.
Tell him to speak to his brother and set out that SIL's behaviour is unacceptable and needs to change.
Tell him to tell him to tell her to behave in a civilised fashion.
No scrounging free food.
No vulture behaviour over other people's presents.
Shouldn't have to ... Yet here we are.
If they fail, on the day, imagine she is a dog attempting to steal food off people's plates, or a toddler trying to steal another child's present.
Find your 'NO. BAD GIRL. That's food for everyone / that's x's present, not yours.'
NO can do a lot of work by itself.