I am very worried both my sister and parents will not be happy about this news. My sister has always been the favoured child
When you are ready to tell them, tell your parents. Give this to them as good news, wonderful news !
and don't mention in any way your sister's wedding.
Keep the two things separate. Because they are. And try not to "borrow trouble." Go into the conversation with the sole thought that it's wonderful news for you, and you hope they'll join you in being happy and excited.
If your parents start to go on about the timing and your sister's wedding, then try to stay very calm. Let them talk themselves into a corner, if they do ...
If they go on about you and your baby being a problem, repeat it - calmly - back to them: "So me not being able to attend a wedding (a social occasion) is a problem that's of more importance to you than my pregnancy?"
Let them tell you what they think.
Oh, and don't try to trump your sister's wedding with stuff about "first grandchild." Just ask them to join you in your happiness and excitement.
Please don't play your sister's game of demanding that everyone else treat you as though you are the most important person.