Tell your husband you are quite happy for him to look after his sister's children,
but at her house, maybe when he has to spend 7am to 8pm,at his sister house a few times it might become apparent how much it is putting upon your family life, and tell him to take your older ones with him,
Sisters can be very controlling over brothers, she probably feels totally entitled to this childcare, as for her bulling, you need to put your foot down, if she is prepared to be so rude and horrid to your face, slyly when others aren't out the way, she will probably be happy to talk about you like that in front of the children,
the tactic she is using, saying really nasty things when others are out of ear shot is common, usually to get a full on reaction, so to others that don't witness the action, it appears as if you are totally unreasonable,unbalanced and difficult,
The next time she tries this on, asks you if you have put on weight, respond by , shouting through to your husband, " Darling, your sister wants to know if I have put on weight, what do you think?"
what ever she says, shout it out exactly and ask for opinions, she at the moment is counting on you not to publicise, but to complain to her brother, stirring and undermining,
How often does she have your children for a day?