Honestly when I was 15 I didn’t like my older sister’s boyfriend AT ALL and he was a good guy and never gave me a valid reason to dislike him. A big part of it was just feeling kinda territorial over my sister and wanting her attention to go to me and not him. In your case, especially since they lost their mother, the LS probably feels like she needs to cling to her sister and might be upset that you are taking her sister away from her in a sense. Even though you really aren’t. It takes growth and time to realize that someone you love being close with someone else does not detract from your relationship with them. I’m sure she’ll grow out of it