Honestly, if you've given/loaned money several times in the past, you've already established and reinforced your SIL's expectation of you--an available source. If she's paid you back, even if a bit late at times, then in her mind, she's good for the money, and you have no reason to distrust her. Also, you indicate that her behaviors haven't changed over the past several years. If she hasn't changed, and you haven't changed, and your relationship hasn't changed, and she's always paid you back, then she has absolutely no reason to expect you to withhold the cash this time. Upsetting this apple cart unexpectedly will most likely end poorly for your relationship, and she's not entirely to blame. People on this forum complain about professors, bosses, spouses, parents, children, banks, politicians, etc all the time--typically when something was withheld that the complainer thinks he's entitled to. It's all about perceptions.
Decide now if this is the last time. Then give her the money with a thorough explanation that this is the last time, why it's the last time, and offer to help her make this the last time she even needs to ask. Then you've given fair warning for the future. Although a "leech" (terrible word for even a bad SIL) might ask again anyway and still get mad if you refuse at that future point in time, you have given reasonable notice, you've completely exonerated yourself from accusations of unfairness and unexpected behavior, and you've set a precedent of level-headed thought and process that should appeal to your wife more loudly than the hissy fit your SIL throws. Be kind to your SIL. Win with your wife.