I'm thinking of doing something with my sister's underwear and I need people to talk to me about it.
Is there anything that could stop you from doing something to it?
I need someone to hold me.
It's not unusual to have urges the other people would consider perverse, but there are certain boundaries that you don't cross. Handling your sister's underwear is inappropriate and considered incestuous, which is taboo. It's good that you're reaching out for help, and if you can't resist these urges then you need to seek professional mental health assistance.
...Is touching another man's underwear bad?
I think you should just stick to your own undergarments. You shouldn't act on urges that will make you look bad. Touching other people's things is disgusting.