Hand/arm tremors

My wife of 78 years of age (white) and is ashmatically but 100% controlled with her daily inhalant. She  has been suffering from a right hand and arm tremor.  The tremor is all the time she is awake but stops whilst sleeping.  She has had this for approx 9 months.  Has had a brain scan and the report is normal.  She has had a variety of medication which have all made her very unwell and has had to stop taking them. The present one is Gabapentin         which she has taken on a reduced dosage and has now worked up to a full dose of 100mg 3 times a day which she has been taking for just over 2 weeks.  This has had no affect on the tremors in fact recently it has got worse although this might be due to using the hand/arm more recently (She has been involved with making a wedding dress!!).  The tremor is frequently very marked and there is nothing she can do to control it.  I believe that a Beta Blocker gets good results in these cases but due to her ashma she cannot take it.  Any suggestions as to what could be done.  She is a very active lady and finds the tremor very inhibiting.  It is her right hand/arm which is affected  and finds it very difficult coping with her left hand