So the day before your sister is due to get married, she discovers that her husband to be has been sleeping with milkman, and the wedding is off, and you .........................,
or
you wait and then in eighteen years time you have another two years of a stroppy teen going through A levels, rather than a nice quite house to pack in as you and your husband get ready to go on holiday,
or
you have problem TTC, and you forgo having another child,
or
suddenly a parent develops a serious illness , your husband and you are run ragged lending support,
so many variables on life, some of which might really stop your plans,
I very much doubt a baby will interfere with a wedding, a wedding is one day, and a baby is a part of your family forever,
I couldn't imagine finding a sibling extending their family an inconvenience to my nuptials, I would be thrilled for them.
If the time is right for you and your family, do what is right for you.