Out of interest, how long had he known your mother before marrying her? Longer than 5 years, I'd assume by your assertion that 5 years is a short amount of time. Granted, by today's standards it is. Has your father made a will already? If so, marriage changes nothing. If your father hasn't made a will get, it would be prudent for him to do so - not for your sake but his own.
Your wording, incidentally, does lead to the following summary:
You don't object to your father's relationship as long as he doesn't marry, however remarrying is an insult to your mother's memory, and you're concerned about your inheritance.
My mother died several years ago. I'd be absolutely delighted if my father, who works hard and lives by himself, managed to find someone new as my mother insisted he do before she died. Would I be happy if he remarried? I couldn't honestly say, however he's made his will, and he's stated that it will NOT change, no matter what his circumstances. In point of fact, that doesn't actually matter to me at all. The only concern I gave him was how often I'd see him if he started seeing someone else, as I don't want to lose him at all, it's one of my biggest fears which I know will one day be realised. I wonder if your mother would want him to move on with his life or if she, too, would prefer he lives a lonely existence just to protect your inheritance. (This part of my response is based on the wording of your post, on which more emphasis, I felt, was placed on inheritance than your mother.)
However, you're not wholly wrong to be concerned, depending on your father's state of mind.
I presume your father has his wits about him, in which case the tale I am about to impart shouldn't matter, but...
A friend of mine's father wound up in hospital. By all accounts, it seemed he was completely compos mentis, however his nurse somehow convinced him to get a divorce. She then somehow managed to convince him to marry her. And then she somehow managed to get him to either change or destroy his will. This was within the space of six months. He died a short time after this, leaving his wife of 30+ years and his son with nothing, and there was nothing they could do about it because he had appeared to be compos mentis.
Now, an additional question for you: you say your sibling and you don't like her. Have you thought about why that might be? You've given the excuse to your father about ultimately being disrespectful to the memory of your mother. Is that the basis of this dislike? Or is there more to it which has nothing whatsoever to do with your mother? Might it be the way she speaks, the way she carries herself, the fact she's nothing like your mother?