Sigil "likes" form code just fine. You just need to make sure it adheres to the more strict xhtml rules and not the lazier html (Those are EPUB requirements, not Sigil).
Also, although it will render fine in Preview, form code is not likely to validate (nor be at all useful) in EPUB2.
The following code renders (and validates) fine for me in an EPUB3.
Code:
<form>
<input type="radio" id="male" name="gender" value="male"/>
<label for="male">Male</label><br/>
<input type="radio" id="female" name="gender" value="female"/>
<label for="female">Female</label><br/>
<input type="radio" id="other" name="gender" value="other"/>
<label for="other">Other</label>
</form>
Hooking it up to javascript for functionality is entirely up to you. Interactive ebooks is not at all my forte (or interest). Nor is it a Sigil topic. It's an EPUB topic