I'm only 5 feet tall but weigh 105. I never used to worry about it, but my sister who is 10 years older than me looks a lot thinner than me. So does her 6"3 husband. I'm a teenager so it's clearly not "puppy fat"?

I wouldn't consider it fat.

-Don't let others appearances influence you about your own. You are entitled to your own physical condition and having your own type of unique appearance.

-Make sure to know the right definition of fat.study more about imbalanced weight, because you might even just being sensitive on yourself and overwhelming while you're good enough already.

-Since you are a teenager, there's high possibility about that sensitivity. It's a phase and you'll pass it. You just have to embrace who you are as a person and know that you're great enough even with few sizes less or more. These changes won't decrease your value.

-But simply, if you're not pleased with your gaining sizes. You can take a look at your nutrition and lifestyle and make some changes. Although you must share it with a trusted person, for instance your mother, who could understand you , help you, guide you and manage this through right along with you.

Five feet tall and 105 pounds is not fat.

You are at an ideal weight for your height. And quit comparing yourself to others. There are so many factors that go into one's build, you cannot make a true comparison.

I'm sure your weight is fine.  Don't compare yourself to anyone else.  If you're concerned though .. Ask your doctor.  You are a teenager and still growing.  You should be concerned about being healthy and feeling good.

Endomorph, mesomorph, ectomorph, Small boned, large boned, genetics, current age, age plus a few more factors involved.

Nothing to be embarrassed about---talk to your mom and sister.