I'm busy. I have dear, life long friends whom I only get time to see once or twice a year, for a few hours each time. You are visiting every 3-4 months, so 3-4 times a year. And she is spending a few hours with you each time? She is making A LOT of time for you in my opinion.
If you want more time with her, you might have to compromise on what you do with her. Sometimes my best friend and I catch up while grocery shopping - we might not have time to hang out and get our weekly chores done, so we combine them, an you do something like that? She might simply not have tons of time to sit around chatting, but might have time to catch up while doing something else.
Alternatively do you plan for doing something fun while visiting? Some people like to sit and talk to catch up/visit with family. Some people thing that's painfully boring and a terrible waste of time. But going to a museum, or to hear a speaker, or to a concert, or a local fair, or a sporting event, or whatever is fun for them. Or if you are always doing something when you visit, maybe she wants to just sit and chat.
It might not be that your SIL doesn't want to visit with you, it might be she doesn't want to visit they way you like to visit.