I can't take Vit D supplements either and also have a low serum level. I get what I can from food but can't tolerate fish, and many D supplements have fish oil. It is possible you may be able to tolerate a synthetic Vit. D (calciferol), I've seen some online
The relationship between serum calcium and bone calcium is complex and a person with osteoporosis usually has a normal serum calcium level.
You and your mom may have a maladsrobtion problem and cannot adsrob calcium and Vit D from your foods. In this case you need to rule out gluten sensitivities, especially Celiac Disease, which is quite common and often misdiagnosed or undiagnosed.