Anne,
No doubt the dream is addressing the relationship with your bf's mother. But I get the sense the dream is as much about nurturing aspects, being accepted by a mother. that begs the question, 'what was the relationship with your mother'? The importance of being accepted by his mother could be addressing both issues with the mother representing emotional/physical nourishment. Are there unresolved issues with nurturing aspects in your life {early life especially}, with your mother? All dreams have two or more meanings/applications and my sense is this dream is primarily about nourishment {mothers} and being accepted by your bf's mother may be a substitute for a mother's attention and nourishment, being accepted by both his mother and your own mother.

If there is not an issue with your mother then the 'other' application {besides the acceptance of his mother} then look to your mother's {and yours} relationship with your dad. The dream does reference an undeveloped masculine aspect {within you} and that could be due to the father relationship. If there was a negative relationship between your mother and father then the importance of being accepted by his mother would address the issue of relationships between male and female {as noted in at least one instance in the dream}. The 'mother-in-law' reference may be about 'legal' {law} or proper relationships. This dream language and amplified interpretation could point to this:
-tells about her son/likes and dislikes-undeveloped masculine aspects that are positive and negative
-about him being musical-developed harmonious integration of masculine aspects/creative aspects

Summary
As I stated, and what fits with your current life {emotional energies} is the relationship with your 'possible future' mother-in-law. The importance of 'being accepted' is paramount in your life. But with all dreams the deeper issues of your emotional life is being addressed as well. The mother-in-law would be symbolic of something other than your bf's mother. And often mothers represent emotional/physical nourishment, something that is vital in early life. If not there as a child then the likelihood is there will be a search for that nourishment throughout life. That could make it even more important there is acceptance by your bf's mother, she could be a substitute for your mother. Of course if your relationship with your mother was strong in 'all aspects' then there would be something else that his mother would represent {there may have been something lacking even if the relationship was 'OK'}.

Amplification if Images and Actions
Title: Mother-in-law
-unresolved issues related to inner/outer nurturing that needs resolution

-was supposed to fly somewhere-extending your latitude/actual experience
-bf going to be there-your masculine aspects/actual bf
-arrive few hours later-masculine aspects that are not completely developed
-time to spend/not sure what to do-coping with pressures/unsure about some aspect
-walking down street-progressing in life
-mother-in-law suddenly appears-new and unexpected visible aspects related to nurturing/actual experience
-start having a pleasant conversation-inner communication
-tells about her son/likes and dislikes-undeveloped masculine aspects that are positive and negative
-about him being musical-developed harmonious integration of masculine aspects/creative aspects
-how he plays guitar-playing with emotions
-him being hard working-working hard to live up to masculine expectations
-standing on stairs-higher level of understanding {aspects within yourself}
-makes wreath out of leaves-bonding and growth over time
-mentions something about marriage-union of opposing emotions
-gives her blessing-positive outlook of nurturing aspects
-we both run up stairs-issues that are competing to be understood/resolved
-asks me if i would like some food/I say yes-in need of emotional/physical nourishment
-we go to Starbucks-critical desirability/interest to nurturing aspects
-sharing a snack-allocation of collating nurturing
-happy she wants to share it with me-sharing of feminine/nurturing aspects
-feel included/closer to her-positive emotional aspects related to nurturing aspects

Jerry

Age & Gender & Location {Required}: 67 Male Cocoa, Fl