This is your husband's issue, not yours. Throw it in his lap. He has helped her for free before, so he's kind of created this situation. He should draw a line between the family and his profession. He can be light but firm about it, saying, "I'm sure you understand that doctors and lawyers and counselors and real estate brokers get approached all the time by people wanting free advice, and as much as we'd like to help, it's just not feasible. I'm sure you understand."
It's not clear to me if you all need the income from helping out the family and therefore you want to continue to use family members as clients. It's okay if everyone's on the same page, but it can create hard feelings. If this MIL is aware that he provides services for other family members (I realize they are on the other side) but not aware that he charges them, she may think she's "entitled" or even "invited" to consult with him. It's possible that she considers this a way of "reaching out" to a semi-estranged family member because it's the only way she knows how to communicate, although it's also possible she's just looking for freebies or attention, or both.
Your husband needs to tell your father's wife that, at home, he's off the clock, and he just gets so many calls from friends and family looking for free advice that he doesn't have a second to himself anymore. He can offer the MIL an appointment in his office to discuss the matter, and tell her his hourly rate is X. Otherwise, he can give her a referral to another professional in the same field if she'd rather keep it outside the family and on a strictly professional basis. Which does she prefer?
If she calls your home, you simply say that your husband (or his firm's secretary) handles all the appointments for professional advice, and you just handle social invitations. Give her the number of his office in a firm but warm way, just as you would any other acquaintance with whom you aren't going to get into a confidential discussion. Then change the subject and ask her how her garden is or if she's seen a new movie. Be friendly but on a completely different topic. If she persists or calls again, just repeat the same script. Try not to get aggravated because it builds a wall. Just plead no knowledge of his schedule, and say, "Oh, when my husband comes home, he's all in 'family mode' and he tries so hard to leave his stressful job in the office where it belongs. He spends all day dealing with things and he just doesn't want to handle them at night or on weekends. I can't blame him. So we make it a point not to handle his professional responsibilities in the house. I'm sure you understand that." If she persists, you just say you don't know and she needs to call him in his office.
People can't take advantage of you without your permission.