Why does my 5 month old have dents on her 2 index fingers. She is teething at the min so I thought maybe she was biting them. Can u help?

Offer an alternative. Hands in mouth definitely can lead to certain scars and or "dents"- it provides and alternative pain pathway. Teething rings and supersoft baby tooth brushes utilized 3-4 times min daily may assist you with this situation.
Finger sucking? It is very possible your 5 month old is sucking on those index fingers which is causing the dents. This is not age inappropriate behavior. You may consider offering the pacifier more; however, as I indicated, this is not age inappropriate. I would start getting concerned the year of the start of kindergarten. Yes, this may continue for a time....
Not sure......... Without examining the child, it is not possible to say exactly what these "dents"are. If your child is happy and does not act sick, it is very possible that these are the results of biting on the areas. An exam by her pediatrician can probably clarify the cause.