Thank you for sharing your dreams with us. I can see how disconcerting that they can be. Please take everything that I share to the Holy Spirit as He Alone gives the accurate interpretation.
The subsequent narration and questions here are not intended to embarrass you but must be addressed. Feel free to respond or take them as rhetorical questions.
May I know your marital status? Do you struggle with lust or envy? Do you feel that your sister-in-law may be courting too soon or fear that might be the case?
I believe that this dream might mean that there is a lady (literal or representative of someone or something) that either you do not really like her teachings/guidance that is being provided to you and it doesn’t sit well (vomiting) with you. The question you need to ask yourself is why YOU feel the need to “feed” on the messages/teachings/guidance/counsel of this individual. Alternatively, this lady’s messages/teachings/guidance/counsel may have mixed truths (milk and mucous). Finally, this “lady” either literal or what she represents may be a negative influence among men and it does not necessarily have to be sexual in nature.