The <br /> tag is an inline element and cannot be placed outside block elements. Faking it by wrapping everything in a div isn't a solution. The epub is badly coded.
In Sigil, select
<br /> [linebreak] <p class="para1">
press Ctrl-H and put <p class="parabrk"> in the replace box and press replace all. Then add
.parabrk {
margin-top: 1.2em;
margin-bottom: 0;
...
}
to the css.