My cat NEEDS help please!

At my house there are two cats there is my cat, Whiskers, and he is male, 8 years old and a little tubby (hes lost weight and is now a healthier weight) and there is Tinkerbell, she is female, she is 3, and really skinny. When i pick her up i feel her bones its so sad and we have food out for her all the time and we see her eating quite a bit, but not near as much as Whiskers. She had a 3 foot long tapeworm and had a cold of some sort when she was young shortly after we bought her and I don't know if this has to do with her low weight or not. But she also is a very picky eater. Whiskers will eat anything we give him and she will not. We tried many kinds of foods and even human food but she doesn't eat much of it. We give the both of them about 2 cans a week but Whiskers tends to steal hers, we try to stop but he does sometimes because she has no desire to eat it. I don't know what to do and anything will help and if there are any questions that need to be answered just let me know and please help me i don't want her to starve.

She's not likely to starve if she will eat the food you put out.  Some cats are just skinny.  We have 3 that are tubby, and one that is very skinny.  Just metabolism differences, to some extent.

Like MrBlanche said, if she's eating, she won't starve. The only time you really need to worry is if she stops eating entirely; cats can get fatty liver disease pretty quickly from that.

If you just want peace of mind, I would take her to the vet for an exam, just to make sure you're not missing anything. She might just have a little bit of acid reflux that makes it uncomfortable for her to eat, I know a few people on here who give their cats some Pepcid A/C and it helps their appetite. But don't medicate your cat with anything, especially human meds, without your vet's consultation.

Welcome to TCS, and good luck with your kitty.

Have you checked with a vet that she's actually underweight? She may just seem boney compared to Whiskers. Cats are naturally very slim animals.