Advice for dealing with very difficult family member please?

Hi my husband and I are having a difficult time with his aunt age 74 ( she is actually his second cousin but she likes to be called his aunt and expects our baby daughter to call her grandma). She is a difficult customer because she is as sweet as pie one minute and then can turn on you. She was widowed last July and we were with her and her husband every step of the way throughout the long and painful battle with cancer putting our own lives on hold and trying to juggle jobs, a 9 month old baby and my spinal disability. My husband has always been very close to this lady. She and her husband are very wealthy and have helped us out financially. However, she refused outside help during her husband's illness and now her mother age 96 needs assistance again she refuses outside help and lays the burden on us and on my husband's mother by playing the victim card. We have always visited them kept them company etc dedicated our spare time to them and even more so since the death of the uncle. She expects us to be omni-present not taking into consideration our own lives at all and if we dont do what she wants there is a barrage of subtle emotional blackmail. It is getting too much we have spoken to her and she negates everything saying that she never interferes and that we are selfish like the rest of our generation and then it's waterworks and she backs off a little. the problem is when you do what she likes or agree with what she says she is genuinely lovely. How can we diplomatically but firmly deal with this situation: my husband and I both agree that it is way over the top. She is independently wealthy but doesnt see why she should have strangers in the house when the family could and should help. Any sensible advice graetly appreciated. Thanks