At the outset, move away from the language of blaming; it makes it hard to put things in perspective.
You’ll just have to be patient and you need to give yourself a break
One of the marks of maturity is to gain skills to deal with conflicting emotions and process them with your lover.
If I were you, I would not feel guilty about achieving orgasms the way my body best knows how.
It would be helpful to urge your sister-in-law to find a way out on her own from the marriage or the issues that she faces with her husband, your brother.