How do I befriend my sister-in-law? We are not the best of friends, but I want to maintain a cordial relationship with her. Any suggestions?

If you had mentioned whether you both are of same age group or not would have helped a great deal in answering the question close to its best. Anyway, I think that in any relationship, whether it is your MIL, SIL, sister and especially the females, you need to give them space. How much space, you will get to know once you deal with that person. Here, if your SIL and you are not fond of each other and still want to maintain a cordial relationship then I suggest do not be opinionated in front of her. Let your point of view on things take a back seat. Let her have the last say, even the first, if that doesn't hurt your ego much. Be a little cute sycophant and live happily.

I know how you feel. I don't particularly enjoy the company of my SIL either. She's very competitive and at times, condescending. So whenever I see her, I don't talk about myself or my life but instead I let her go on an on about her "perfect" life. Just be patient and stick to neutral topics. You can also try going on double dates with your husband, his sister and her partner.