I know that Multiple Myeloma is cancer of the blood plasma, but does it also affect the bones?

My aunt, three years ago, got this weird sore on her foot that wouldn't heal. She went to the doctor and he told her it was due to complications from diabetes, a condition she didn't even know she had. The wound never healed and she wound up having to have her leg amputated below the knee! While in the hospital the doctor put her on medication for diabetes. Well, she kept getting dangerously low blood sugars while on the medications so he pulled her off. Later, it turns out she never had diabetes to begin with, and her blood tests showed signs of cancer. She saw an oncologist that diagnosed it as Multiple Myeloma. The day right after Xmas last year, she fell while trying to transfer herself from the toilet to her wheel chair and she broke her hip. She had surgery and was sent to rehabilitation. She developed a new sore on her other leg and it never healed. She had that leg amputated 3 weeks ago! Is it the cancer that is making her bones so weak? I don't get it. I thought this type of cancer was only blood related. I'm really scared that the cancer will get worse and she'll have more complications.