Is there anything your Mom does (for example food shopping, or having coffee at a certain cafe, or some kind of hobby) that you could accompany her to where you would be alone with her? I am sure if you catch her on her own (in the garden, laundry room etc.) she will be willing to listen to you.
If you don't get a chance to talk to her during the day, can you sneak back up after bedtime and catch her away from your siblings?
While such things can be damaging to the self esteem it's also a health issue, as large and exposed moles can bear a greater risk of developing into skin cancer, so it's understandable that you are concerned about them - good luck!