As a woman, I suggest you err on the side of caution. Essentially you are going out with your SIL's very best friend, in the whole world (next to her husband). That translates into anything bad you do to your date, you are also doing to your SIL. Literally, that is how she will take it. If you treat her bad, your SIL will hold it against you, for a long time to come. She'll try to triangulate your brother into joining her feelings against you. Seriously. Whether or not your brother would side with you or his new wife, does not matter. Just do not even put him in that situation.
See the woman for drinks and have fun. Be super clear that intents are conveyed clearly. No drunk ONS. She is not one to pump and dump. If seeking a possible LTR...then check it out. When you are clear she is not a match for MUTUAL fun or more...be really straight with her in respectful way. Realize when you dump her/ move on whatever words you say or text her are going to repeated to your SIL, your brother, and likely more. Always be straight with her. Absolutely do not ghost her.
Family and holiday gatherings will always be around. Don't let the desire for some quick fun, create hurt feelings, that get pinned on for you, forever more. Again, I say enjoy your time with her but with this one err on the side of caution.